"We make sure the kids are looked after - all of them- taking them on holidays and paying for driving lessons etc."
That's irrelevant. He should do this for his children.
I still think the only way he can deal with this is via the legal route, however (with your last couple of posts) it is sounding rather more complex.
"He didn't pay anything officially to her for the house, just maintenance into her bank"
It is very possible that a court will consider the maintenance payments he paid into her bank as child maintenance. It will certainly be irrelevant that she did not go to the CSA for assessment ... neither did he.
Therefore, in final settlement it may be that a court takes into account that he hasn't paid into the mortgage of the house for the last 14 years, just the first x number of years of ownership of the house.
As someone mentioned earlier the court may well take into account her reduced earning and career development capacity, due to being the main carer of their children.
Add to that the fact that it is an endowment mortgage, not a repayment one (and she could well be right in saying that the endowment now fall far short of the original loan, quite common). All in all, it could be that your DH actually does stand to get significantly less than he hopes for.
In the absence of agreement, the legal route is indeed the only way to sort this out. It is likely they will both end up with significantly less than they would hope for.
I also think your husband should keep in the back of his mind that this is still his younger daughter's home. Very hard for a 19 year old to get their own home nowadays, even when working. Will she be able to live with you when she becomes homeless?