This seems to be an example of the [XY problem][1]. You say: "trip is hard, friend's girlfriend is overweight, how can I suggest that she stay home". Issue here is that you are not the best judge of what is "overweight" and what is "hard" and how these two are connected. Don't try to solve wrong problem.

You real concern is that trip is hard and requires experience, and you suspect that someone might not be up to the task. Since you are the organizer of the trip, it is your responsibility to match people in their ability. Also it is your right to deny people you don't know joining, after all you take on responsibility for everyone's good time.

So proper question to ask your friend would be: "that's a hard trip, I am not sure your GF has the required experience since we never hiked together. Are you sure she is up to the task?" However, if you do not trust your friend's judgement, then you can't use his recommendation, you have to decide yourself agains taking newbie.


  [1]: https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem