Hello and thank you for the question.
Before this breakup, did your niece struggle at all with her weight? Was she considered within normal weight range, before the break up? When is the wedding date?
Okay, thank you for the additional information. Actually having the wedding date as a motivation is really helpful, plus it's far enough away that she has time to lose some weight, without going on a crash diet, etc.
The best way to help her is to simply offer up suggestions of what you know has worked for others. Such as, I know just how you feel. One of my good friends, also wanted to get slimmed down, and she joined (insert here) weight watchers, or jenny craig, or the gym and ate fish and chicken etc. and really lost weight. In summary, it's to keep it light and upbeat and offer solutions. If you want to lose weight (maybe you don't) but if you do, you could even offer to be each other's support partners etc.
I do think that it could be really helpful for her to join something like weight watchers where they support and encourage and she has a plan to follow, but it's also reasonable. So again, you say "I was thinking about you, cause one of my friends ,or neighbors (whomever) joined weight watchers and really slimmed down and also enjoyed the process etc. I wanted to tell you cause you had mentioned being worried about the wedding etc. and wanting to slim down.
That is all you can do really. The rest is up to her. Sounds like she was quite depressed after the break up, as 50 lbs is alot to gain. Exercise is also so key for her emotional wellbeing and weight, but most of these diet programs of course encourage this as well. Best of luck. Also, coming from an Aunt, advice is often more welcome, than say from a mother. :) Please click the green ACCEPT button, so I'm credited for my answer today. Thank you.