To be honest, apart from the pee stop issues, I don’t think there are any gender-specific things I can think of. I’ve not got problem with HR straps and haven’t heard of anyone that does. I’d urge caution with clothes though, fit (and style) is very individual, and she’ll probably need to find out what works for her. I’d take her out on a ride and look for things she doesn’t have.
If your looking for gift ideas…mtb coaching sounds good, and shoes? Some nice 5-10’s maybe…