It's interesting that for all your claims of financial independence and saving vs her sitting on her arse, you want this man's income to secure a mortgage just as much as she does.
In amongst the bitching about her, maybe stop and think that she probably hates this situation too.
You portray her cackling over having played a good game. Hardly. I think she's ignoring the situation because she's confused, scared, badly informed - and hasn't a clue what to do for the best. She's probably very frustrated - you have more in common that just your choice in men!
She may have ignored solicitor letters - but then your husband just gave up too, without ever really pushing it very far. Like - even finding out how much he owes
The pair of them would drive me batshit!
All this talk of court though...
He needs to start with the basics - find out what they owe.
If the endowment is due to mature in 2 years time, and only has a £3K shortfall, that's great.
He needs to get all the financial information. Has he even googled or posted on MSE asking who were the main endowment providers in 1994 to see if one jogs his memory? Although other posters have given great search info.
He needs to speak to a solicitor to start the ball rolling to ensure that the endowment policy can only be used to pay off the mortgage.
With the caveat that I'm not a solicitor, I really can't see a judge ordering a sale when she resides there, when the endowment would end the mortgage in less than 2 years time.
In your position, I would look at buying a house with your own mortgage and then upsizing later. Of course, if it's your mortgage, make sure your have a deed of trust and it is very legally clear who owns what share of your house.