Clearly you are being very economical with the truth here. £100 per month is too much, but you supported the mother by paying half her mortgage?
I was lead to believe the bursary was maybe £250/£300pcm in which case I was absolutely happy to help her out. Between that and her job it's a lot more,
It doesn't sound like she mislead you though. It sounds like her bursary is £250/£300 per month, but then her overall income with her job is around £700.
How do you know she contributes absolutely nothing to the household? No food, no bills, no rent, no anything? Especially considering her mother struggled so much financially, you had to support her?
And do you actually know her outgoings? I would assume with a brand new car, she is probably paying out at least £300 per month with finance/loans, insurance + tax (unless she had a lump sum of money to buy outright but I'm guessing not). So I doubt she actually has £700 disposable income.
Ultimately it is your DH's decision if he wants to give his DD a token amount whilst she studies.