How to tell a teenage girl that her body odour is very strong and she should do something about it?

We use to have a colleague who has a bad body odor whenever she started sweating... When we confronted her,she's embarrassed but then she's thankful that we wanted to help her,it turned out she is using a perfume as a deodorant!so we advise her to use deodorant and the next morning she doesn't smell bad anymore!☺