It’s none of the gymnastics coach’s business what she does outside of the gym. I would encourage her to refrain from mentioning rugby at the gym and, if asked, she should make it clear that she is not seeking feedback on that topic. She should definitely shut down any discussion about her body composition and whether she is “putting on muscle”. That kind of talk is totally inappropriate and stupid. If she does not have the self-advocacy skills to set boundaries with the coach, a parent/you may need to schedule a meeting with the coach and their supervisor to help facilitate a discussion. This is total BS and should be treated as such.
Furthermore, As a 16-year-old level 6, she most likely nearing the end of her competitive gymnastics career. Rugby may offer her more opportunities to continue playing after high school. It’s very important for gymnasts to have other athletic interests, so they are not left with nothing to transition to when gymnastics ends.
Sorry that he coach sucks. Hopefully this will provide her an opportunity to learn about boundary setting and improve he self-advocacy skills.