I agree with your mom. The t's focus should be only on you. Even if it's "just walking" on a treadmill, you still have to pay attention to what you're doing so you don't accidentally step off the side or something (my h has actually done that while watching TV and walking on it) so the t's attention would have to be split between you and what she is doing on the treadmill. So yeah it would bother me. I'd be like, I'm not paying you to work out. I'm paying you to be present for me.