well the doctor in this video said she'd be reducing her lifespan, which is probably true, but if this makes her happy would that not be an argument of quantity versus quality ? besides who knows what future medical advances might bring.
My advice would be to do a genetic test (it's about 100$ to 400$) and see whether she has a genetic weakness. If the test doesn't show any genetic related weaknesses that being overweight would aggravate, there shouldn't be a dramatic lifespan reduction.