Depending on my weight, I suppose I would gain some weight or stay the same. But I would also have a talk with my best friend. If being over weight makes him or her happy. I would support her. But if they feel sad about their weight, I would help them lose weight. Although I would want to make him or her feel good, so I could gain a little weight, and then maybe we could work on losing it together.
(Please note I wrote this based on my own opinion)
Hope this helps!!