My weight goes up dramatically when I change medication and I have to lose it again, so I have done that about 3 times now, gone from about 14-15 st to 10-11 st in just a few months.
I find it easiest just to make a start and go hard at it, 1200 cal or less and 5:2, cutting down booze and carbs plus exercise. Your friend may just suddenly have become motivated (maybe seeing an unflattering photo, a favourite dress didn't fit, or a health scare, including covid-19) and taken a similar approach.
For me, 1-2lb loss a week would just be too slow when I have several stones to lose and I would lose momentum. I think when it's a considerable loss, the benefits of getting to a healthy weight are what spur people on to act fast once they've finally made the decision to do so and outweigh the risk of cosmetic issues like stretch marks.
Plus I find it easier to get used to eating differently rather than just changing things a bit, the lines are clearer and easier to stay within.
All in all, your friend might just have made a really big effort and gone all out to change her habits. If you're still concerned, check in and see how she seems.