I am SO glad you asked this, because I know it’s something other people can relate to and have wondered about. It can be embarrassing to talk about, but here’s the deal: when we start going through puberty, our body chemistry changes. We start sweating more, and smelling differently, and not everyone realizes this, or knows how to prevent it. Also, keep in mind that while it might be because of personal hygiene issues, like she needs to shower more often or wear deodorant, it could also be something she can’t control, like a side effect from a medication, or because of some kind of food her family eats. No matter what the cause, you don’t want to sound like you’re attacking her.
People can be really critical and I definitely don’t think you should criticize a friend’s fashion choices, how they do their hair, or anything like that. But in this situation, it’s actually getting in the way of your friendship. AND if you’ve noticed her body odor, then others probably have too. Even though it will be uncomfortable, she would probably rather hear it from you than from someone who won’t be concerned about her feelings when they make a mean remark.