Here is ChatGPT’s treatment of the issue.
First, it is important to know that any discussion of interest — and especially “verbal interest” — is a complex and sensitive halakhic matter, and sometimes there are fine details that may vary from case to case. Therefore, if you have a practical doubt or a specific question about how to act, it is recommended to consult a rabbi expert in the laws of interest, in order to receive guidance suited to your specific situation.

Still, in general:

Definition of interest
Forbidden interest refers to an addition (monetary or equivalent in value) that the borrower gives the lender by virtue of the loan itself, beyond the principal of the loan. This can be a direct monetary addition, but also a tangible benefit or service that the borrower would not normally have given were it not for the loan.

“Verbal interest”
There is a concept in Jewish law called “verbal interest” — when the borrower gives the lender special treatment or excessive praise, where it is clear to everyone that the sole reason is the loan (that is, he would not have done this at all without the debt). For example, if a person was not accustomed to greeting the lender or calling him by certain honorific titles, and suddenly after the loan begins doing so in an exaggerated way, with the intention of “repaying” him for the loan.
However, not every expression of thanks or good treatment is automatically considered verbal interest. There is a balance here between natural and legitimate gratitude, and giving exaggerated treatment that is perceived as real compensation.

Is complimenting and praising the lender considered interest?

Basic gratitude (saying thank you, telling friends that he helped you, being happy when he succeeds, etc.) is not considered forbidden interest. This is a human and natural act of gratitude, and in most cases it does not constitute monetary compensation, but rather basic decency. Natural help or responding to requests he makes (such as helping him arrange something in the house, giving him a ride, etc.), when it is reasonable and accepted help between friends, and especially if you would have helped even without the loan, will generally not be considered interest. If you reach a point where it is clear to everyone that you are “subjugated” to him and willing to do things for him that you would never dream of doing for anyone else, and he too feels that you are “compensating” him in this way for the loan — there may be a concern of verbal interest here. Especially if there is a sense that the benefits or help are a “hidden condition” or compensation for the profit the lender could have made with his money instead of lending it to you.

When is it still worth consulting a rabbi?

If the help or service is completely unusual relative to your normal relationship — to the point that it is clear you would refuse, or would never do it at all for others. If something was done or agreed upon (openly or by hint) as part of the loan conditions (“I’ll lend you the money on condition that you help me over the coming year with my garden / my business”). If you intend to give him a significant benefit beyond ordinary gratitude.

Bottom line:
Thanking a friend, naturally mentioning his kindness to others, and helping him from time to time when he asks — usually is not considered interest. This is proper behavior of gratitude between people. The concern for verbal interest exists mainly when a real or exaggerated “compensation” is created for the loan, especially if it serves as a substitute for or “compensation” for monetary interest. If you want to be on the safe side, consult a qualified rabbi and explain to him the exact nature of the help and attitude you want to give your friend, and you can receive calm and reliable guidance.

This answer should have been posted as a talkback to the columns about AI. There’s no answer to the question here.

Here is the AI’s proposed solution to the problem you raised in your answer:
One can understand the Chida’s words as follows:
When the loan is still in effect (that is, the money has not yet been returned), there is more room to worry that any special treatment or exaggerated expression of thanks could be considered “compensation” for the loan — that is, “verbal interest.” Once the debt has been repaid, and you no longer owe him anything, expressions of thanks or honor no longer stand in the context of “compensation for a loan.” Therefore, specifically after you have finished repaying him, you can speak well of him and show gratitude broadly without entering into a concern of interest.

But why is there no same prohibition of interest after repayment, unlike monetary interest?

Monetary interest is an explicit halakhic definition: any monetary addition to the principal that is given because of the loan — at the time of the loan, during it, or afterward — can be considered interest.

Verbal interest is different: we are talking about special treatment, an expression of honor, or praise, which is not necessarily worth money. As long as the debt exists, the special treatment can be interpreted as a kind of non-monetary “payment” in place of interest. But after the debt no longer exists, the thanks and honor do not “complete” the loan or compensate for it, and therefore the concern for interest falls away.

In any case, the very fact that the Chida permits thanking a person after repayment helps solve the practical difficulty: there is no reason a situation should arise in which you cannot show him gratitude and say hello to him for the rest of your life. But of course, in any discussion concerning interest in practical Jewish law, it is worthwhile to consult a halakhic decisor expert in the laws of interest, since sometimes small details carry great significance in defining the prohibition or the permission.

I don’t see anything here beyond what I wrote. It still isn’t fully clear to me what the difference is between money and verbal interest. With money too, giving the money afterward can be interpreted as disconnected from the loan. Maybe in such a situation there really would be room to permit even money. But I don’t remember a source like that, although the reasoning certainly suggests that once the connection has been severed and it is no longer linked to the loan, there is no interest here. But in our case, it was stated that there is a connection to the loan. He is doing it only because that person lent him the money. Maybe after he has repaid the money, it no longer looks connected, even though internally there is a connection. And maybe such a distinction is relevant only with rabbinic interest, because with Torah-level interest we do not go by appearances. Bottom line, I still do not fully understand this distinction.