I worked for some very wealthy people, it's quite a heady experience to be around money, isn't it. I can see how your DH has been drawn in.
Were I you (and you have probably already done this) I would point out that while she was able to keep her money to make money for her and not having any legal documentation around the loan, any payment or interest terms that it is less likely she would have paid it back at any time. In the meantime she was suggesting that he and you not only pay interest on a remortgage for him having helped her for 0%, she was trying to encourage tying up both your property effectively costing you more money if you needed to sell up and break a trust to move to a suitable house for you future.
Financal problems are an absolute killer for relationships as you well know. So effectively she was willing to leave you both without money, remortgaged paying interest for years to come, potentially paying out more money than need be in putting your house in a trust and causing arguments in your marriage and causing a rift between you so he attends the parties without you and she can potentially make up any lies she likes to get him out of the way, out of work and break up his family whenever it suits her. All so she could have a 40K loan, nah she was hoping to tie him up in so many distractions he wouldn't notice them fading out from his life and not repaying a penny and then sticking him for more money.
I'd ask him exactly why he thinks these are the actions of a friend. And what on earth does he think he has done to cause such hatred.
These people are manipulators, they are parasites on normal working people, she probably thought taking your money was fucking hilarious. I wouldn't be anywhere near friends like this, they are no friends at all.
Stay friends with these people and they will bleed you both dry and break up your family, they are a poison without an antidote.
“Neither a borrower, nor a lender be;
For loan oft loses both itself and friend,
And borrowing dulls the edge of husbandry.
This above all: to thine own self be true,
And it must follow, as the night the day,
Thou canst not then be false to any man.
Farewell; my blessing season this in thee!”
William Shakespeare