I know several people to whom this has happened. In all cases save one ALL spent monies were part of the decree. Meaning the courts usually try and split things 50/50. It's a balance sheet. Just as an example... even though we jointly own our home, because of how imbalanced out 'sheet' is, I get the house outright if we file. And possible tens of thousands more just to 'balance'.
As long as she has a GOOD attorney, it doesn't matter what her husband has spent the money ON... he's spent the money and owes her half the total. So if he put 100k down on the house and 50k on the girlfriend, it will work out that he 'owes' her 75k on the balance sheet. Which may be in form of a check, or it may be in a redistribution of assets (like their own house free and clear... or both cars... or, or, or, or.).
This also happens a LOT (I'm told, at least) with one spouse and an education. Known as the "I flipped burgers to get him through medschool" situation. If one spouse has purchased and education using family money/time they owe the other spouse and equal education AND a percentage of their increased income for x number of years.
It's not that she'd own 1/4 of the house, per se, it's that she's owed 1/2 of the money he spent. Any good attorney will be able to get that money back for her either in actual money, or in assets.