It's not clear if she lives at home whilst studying or if she rents near her university.
If she was in student accommodation, rent is payable every month, regardless of holidays or not using the property and it costs an awful lot more than £100/month.
Assuming she's living at home whilst studying, that in itself saves on student accommodation rent. Presumably that's one of the reason she'd be living at home: to save on student rent.
If she's capable of working p/t and studying for a degree, I don't see why she'd need to learn the value of paying you £100/month. She should already know about budgeting and earnings.
As a general comment (not for you OP as you don't appear to have this idea, it is your DH who thinks this way): I don't understand parents who claim their children need to learn these life lessons once they turn 18. If they don't know how basic life works by 18, what does that say about parenting? Surely you raise your child, teach them life lessons as they grow and you have to trust they will manage it.
They should be aware of household financial situation and volunteer to contribute what they can afford if money really is a problem for the family.
Or the idea I keep seeing on MN, that, somehow, at that stage, they can no longer afford to support their children living in the same house. Might as well swap the 16 and over child for a paying lodger.