Personally, I would have a very difficult time with this!

Who in the heck wants to hear that they reek? Especially if they're very sensitive and/or low self-esteem? That is very painful! That said, they sure as heck don't want people to think that of them. I'd recommend using a very gentle manner to see if you can help figure out the cause.

There are a few possible reasons that I can think of: her environment at home (may not be the cleanest home, and inhabitants lose their sense of smell to those odors); hormonal imbalance (may produce excessive sweat); not regularly bathing or showering; and lastly, not knowing how to properly care for herself (with menstruation).

If you know your friend well, hopefully, you can answer most of these questions yourself. The last two possibilities would require more sensitivity when bringing up the issue. If she has a computer, maybe you can send a medical link that discusses bathing, hormones, and menstruation.

If she lives in a stinky house and is pretty open about it, tell her straight out that you can still smell the stinky house on her clothes. That ought to help.

Best wishes!