I'm really, REALLY surprised she has not mentioned this in conversation to you OP. Are you not very close? Could there be a reason she hasn't mentioned this?
As someone who has struggled with weight all my life, particularly since teens, I just can't imagine a scenario I wouldn't have at some point, spoken to my mother about it.
That being said, the ONLY time I gained this much weight, this fast, was gaining 5 stone in a few months, after having the contraceptive implant fitted. GP & nurses (whom I did speak to about it, as I knew myself I'd not changed anything about my lifestyle, not a bean) absolutely assured me that the contraceptive implant could not cause weight gain, and if anything, caused weight loss in some people. A few years later, I started dropping weight randomly, very fast - had people commenting on it, asking what I was doing to lose weight. Absolutely nothing - I hadn't changed eating habits, exercise or lifestyle in any way. The only thing that had changed, so it turned out, was the contraceptive implant had ran past the time of effectiveness (I was very late having it removed). I have met one other person this has happened to.
I'd not find it hard to imagine that she would be using contraception whilst away at Uni. Whilst it's not typical to gain such a large amount of weight, so quickly, from contraception, it absolutely happens in a minority of cases. For me, it almost as if it 'triggered' my PCOS symptoms, which I was later diagnosed with (whether it would have emerged anyway, who knows, but it certainly triggered something).
There are also other medical problems it could be - cushings syndrome is one.
I don't want to get into the great big 'fat debate' with people, where we all argue that fat people are just lazy and greedy vs medical issue cause, but at 5 foot 2, and speaking from experience of having weight issues, gaining that amount of weight in a year by diet alone is a massive undertaking and not an easy feat, without some other problems at play. If it WERE from diet alone, I'd be really concerned at what's suddenly changed so quickly, to make her diet change that drastically and quickly.
Drinking and a bad diet will absolutely make you gain weight, and weight gain can be surprisingly rapid - this is too rapid. Why hasn't she spoken to you herself, is my other concern.
I'd have no idea how you'd raise this with her given she hasn't initiated the conversation herself. But there's more to this.