@HelenHywater
What the OP is saying (from what I can work out), is she put in most of the deposit. Let's say she paid 100% of the deposit. She talks about it being hefty, so let's assume that means much more than 10% down. Let's say the house cost 500k, she put in 200k . Mortgage of 300k.
The way their deal works, on day 1 she is entitled to 100% of the house. Over time, his mortgage contributions give him equity. It's not clear how they've structured this, but let's assume that he didn't completely roll over and give her the best deal in the world meaning that when she talks about his mortgage contributions it's the repayment plus interest payment rather than just the repayment.
Assume he pays 20k a year in mortgage.
At the end of year 1, she has put in 200k, he has put in 20k. Collectively the house has cost them 220k so far. He's paid 9% of that, she's paid 91%. So the deal then is she's entitled to 91% of equity and he's entity to 9% of equity. She may or may not have her deposit ring fenced, or they may be equally at risk if house price goes down.
Now over a 20 year mortgage, if there isn't something to say that he doesn't ever get more than 50:50 there's a risk her ownership share goes below 50%. It's not definitively unfair but with inflation etc that's not something that would have been a great deal for her and I'd have expected a lawyer to advise against. Although if the deposit is ringfenced as well then it's still a decent deal. All depends quite how big the deposit and mortgage are and what happens with house prices.
Assume no ringfencing. Suppose house prices go up 10% in year 1 and they end up splitting. House is worth 550k. Mortgage (once you take account of costs of selling etc) let's just say is still 300k. Equity is now 250k. She gets back 227.5k (13% increase on her capital). He gets back 22.5 k (12.5% increase on his capital). She's made more than him out of the deal AND she's had a free place to live for her and her two kids for a year - I don't know where they are but presumably that's saved her at least 1k a month.
There are lots of ways to cut how the structure works and a lot of assumptions in the above. Short term I can't see how this is in any way an unfair deal for her. If it doesn't stop at 50:50 then it is risky long term, but we're talking maybe 15 years before that's a real issue and she's not had to pay the mortgage that whole time.