Tricky question and really need more information to say.
You don't mention age of the couple or the children and where these children from previous relationships live. She may not be contributing financially to the purchase or maintenance of the house but is that because his kids live with them and she is looking after them?
So if he leaves this to his kids, as soon as he dies, she becomes homeless - that seems tough if they are married. There are ways she can be protected from that i.e placing the house in Trust so she can live there until she dies if he passes first and then the house goes to his kids?
My Dad left my Mum and remarried, he brought the property to the 2nd marriage and financially supported them both, she never worked and was a housewife although none of us kids or hers lived with them. When they moved house she was put on the deeds.
They had mirror Wills sharing the money between the children from his and her previous marriages when the 2nd one of them died. Dad died first, so the ownership of the house passed to her and we three, his kids, never got to see a penny of his estate because stepmum changed her Will before she died. Her kids got everything.
Having said that, this is my husbands second marriage and he has kids from his previous marriage. He brought significant equity to the marriage so when we bought our first home together, we didn't need a mortgage but for years my income has contributed to the maintenance and upgrade to our home, paid for holidays etc.
We now own the house together as he added me to the deeds, it made me feel valued, trusted, accepted and longterm! We also have mirror Wills so we basically did exactly the same as my Dad and stepmum. We have discussed seeing a financial advisor to look at the split of ownership of the house but ultimately, that could leave the last of us standing, homeless. But it could also help protect us from care home fees.
Without more information, its difficult to say what she should do