First off, I would let your friend deal with the situation. It's only fair: you didn't invite his girlfriend to the hike, he did. Meet him alone and ask if he really thinks it's a good idea for his girlfriend to join. Point him to the website about the hike if you think he doesn't have a clear idea about what he's signed her up for. There's a high chance your friend will know how to convince his SO better than you would, without hurting her feelings.
If your friend refuses to talk to his girlfriend of fails to convince her, you should take the initiative. There are several options you could consider here:
- telling her you don't think she's fit for the hike
- invite her (or rather them) on a try-out to prove your point
- agree with them upfront that you may split up during the hike
- pick a different hike adapted to their skills
(1) will be at least somewhat offensive, regardless of how you put it. As other answers already stated, telling her she's inexperienced is much better (and more to the point) than telling her she's overweight.
(2) is less harsh but requires to invest a full day into convincing someone they're bad hikers. Personally, I would rather spend 5 minutes in a really unpleasant conversation than spend a whole day in a moderately unpleasant atmosphere.
(3) would only work if your friend has enough hiking experience to lead a hike when you split up, or someone experienced will agree to stay with them. Making inexperienced people agree to something they obviously cannot do is a bad thing, and it will be on your conscience.
An alternative to this is that you hike together until you reach a tourist location which your friend and his SO can enjoy while you continue the hike.
(4) should only be considered if you're good enough friends and you're ready to sacrifice your plans in order to have good time together.