For me, it's all a question of context, if the OP thinks that the camera body is the most important thing he has to be concerned about, that's a worry.
However, if the OP has the right skills, knowledge, lenses, flash technique all nailed, the camera body might be his only question mark left, then he should be fine with the 400d.
I would say though, that even if doing it for a mate as a practice, if you're the only chance they have at 'professional' photos, hire or borrow some backup gear. Having to put your hand up halfway through the day to apologise because you have a gear problem isn't good enough even if you're shooting for free - if the B&G are relying on you, you should be delivering 'something'.