Please note, I don't want to be mean or anything and I don't want to go "fat shaming" or things like that.
With my SO and a couple of friends, we planned a hike for this summer. The GR20 is one of the most difficult hikes in Europe and is usually done in two weeks. I just offered to another friend to join us and he has automatically proposed to his girlfriend to come along too. She was there when I asked my friend, so they both said they would probably join us.
His girlfriend is overweight and doesn't have any experience in hiking trips (maybe 2-3 hours hikes/walking, but nothing relevant). I don't think that her coming with us is a good idea. Usually, for this kind of trip, you need to know everyone in the group very well and be sure that they will keep up with the pace. I'm also pretty sure that she can't physically do it right now without some training, but it will be impossible to reach for this summer.
Question: How can I bring the subject up with my friend without offending him or his girlfriend?
EDIT (since conversation has been moved to chat)
I was actually being nice by saying overweight and I know this term could mislead you on what I mean because medically she is "obese". Also there is really nothing personal with her. Once we have started the hike, we can't go back (obviously we can, but it's not the point). Hiking with someone you can't trust or be sure of will cause troubles. It could be dangerous for her, or anybody in the group. I can't risk that, the hike is dangerous itself on its own. The boyfriend is not that experienced too so maybe he can't assess the risks properly for her.