It's a tricky one as yes, she's an adult and can do whatever she pleases.
But that is a lot of weight to gain in a year or so.
It might be worth having a chat about her wellbeing generally. Often weight gain can be as a result of stress, or depression or anxiety.
Or perhaps she's just partying too hard.
Can she cook, and do you model healthy cooking and healthy eating at home?
Does she know how to make cheap, nutritious meals that are both quick and tasty?
Does she have access to affordable exercise options - be that joining a uni sports team, or a gym, or whatever. If she's struggling financially, could you subsidise a membership for the uni gym or similar?
Do you model healthy habits, such as regular exercise (in whatever form), yoga, meditation, gardening etc.?
It's one thing to notice her weight and be concerned, but if she doesn't have tools in her arsenal to help her live healthily, if she doesn't see the rest of her family living healthily - and if she isn't fully aware about the wider effect of a poor diet and lack of exercise then I can understand why she's spiralled weight wise.
I'd strongly recommend you frame your conversation not around body size but around health and fitness. Even better if you and the rest of the family go on a 'health kick' and encourage her to join in. Saying you're concerned about her weight runs the risk of sounding really superficial, but if you talk about exercise, wellbeing and healthy eating then it's much more helpful in opening up a proper conversation about what's going on.
And no, not everyone gains weight at uni. I've always struggled with my weight but I lost a lot of weight at uni because I got really into a couple of sports and I was cycling/walking everywhere rather than taking the bus plus I was absolutely flat broke so lived off lentils and the runty-looking veg from the market.