Is it a red flag if she has no friends?

So yes, many many times, when people arent genuine, nor care about anyone but themselves, they will not have friends. This is definitely a red flag, but do not use it as the ONLY flag - that is all.

Why do friends come and go?

Typically, we form connections with those were closely affiliated with. For example, people at school, work, or a common activity. Once that thing that brought you together changes, then the likelihood of your friendship remaining intact decreases. As life evolves, people naturally grow and change.