This is always a problem when parents lend kids money and it’s not really memorialized by holding the kids to a repayment schedule. That being said, you do not ever want the elephant in the room with your child.
Ask your husband to sit down with your daughter to talk about the money. You can facilitate the conversation, but stay neutral. She needs to say that she owes the money and explain to her father as to why she hasn’t paid it back. She will have a chance to explain how she doesn’t have the money now, or what the problem is. They both have to agree on the repayment. Your husband also has to be honest and say that he felt betrayed, or doesn’t like to nag about money… whatever his feelings. The goal is that an agreement is reached and memorialized in writing (a simple note that says your daughter will pay your husband the money and interest, if that is part of the deal, in increments over a period of time.)
The larger goal is to have them hug and tell each other that they love each other and that they will not let this interrupt their relationship. Now, your daughter has to keep her commitment. If she can’t, then she has to explain that to her father BEFORE she will miss a payment.