It's a tough call. I know there is immense pressure on those of us with "monolids" to get the surgery, so how do you separate whether it's truly an independent decision or not? I also worry about the risks of any surgery for a cosmetic reason, but that's just me being hyper aware of risks.
I ultimately agree that someone choosing cosmetic surgery can still be a feminist. It's about choice and access to the same options men have--and no one questions men who want plastic surgery whether they REALLY want it or if they're bowing to societal pressure. So if your friend wants to get the surgery, I hope it goes smoothly!