It doesn't matter. It's not your problem. If she couldn't find 40k from her own "portfolio" to help her son buy a house, she couldn't afford to help him buy a house.
Why on earth your husband thought it was a good idea to get involved in this is beyond me.
But you need to get something in writing from her confirming that it was a loan and that she intends to pay it back.
Asking to put in place a repayment plan is a great idea because it could help you get some useful written evidence that the money was not a gift.
Your husband needs to stop fobbing you off. Tell him to write to her (text is fine, just not a phone call) and say that she needs to at least start paying the money back, even if she can't pay it all off at once, and suggesting monthly repayment plan where she pays off a set amount each month (e.g. £1000 per month) and then pays off the balance as soon as her liquidity issues are resolved.
Have you pointed out to your husband that if you'd had that £40000 in a basic cash ISA for the last year you'd probably have earned about £2000 in interest? Even if he thinks the £40000 is a loan which she will definitely pay back (lol), what is that £2000? A gift? That you're giving this person who throws fancy parties and takes business class flights because...?
Tell your husband that if he hasn't got some sort of written confirmation from her that it is a loan which she will pay back, and agreeing to a repayment schedule, by the end of next week, you will be taking over communications with her. And if that doesn't bear any fruit she will be hearing from your solicitor.
So again, if he's worried about her having a bad day, the onus is on him to make sure she doesn't start having even worse ones, which she soon will if he doesn't deal with this.