Really hard to find a suitable title, if you find a better one, don't hesitate to edit. I don't want to be mean or anything and I don't want to go "Fat shaming" or things like that. Also, I'm not english native speaker so maybe excuse some terms I may use wrongly.

Context

With my SO and a couple of friends, we planned a hike for this summer. The GR20 is one of the most difficult hike in Europe and is usually done in two weeks. I just offer to another friend to join us and he has automatically proposed to his girlfriend to come along too.

Issue

His girlfriend is overweight and I don't think that 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 ?