She's on her period?
A likely reason for a young woman to have mild bleeding from the vagina, the day after non-vaginal sexual activity, is because she is having her monthly period.
Some women are not regular, and so may have bleeding at other times than every 4 weeks. Another reason is that the rubbing was too strong, but then she would know, due to having some pain. If in doubt, a visit to the doctor can help.