Another reason River knows she can't turn up when Rory asks her to help is because she knows the younger version of her is going do show up and that they shouldn't meet. (Although we've seen disregard for the Blinovitch Limitation Effect in TBB and ACC).

Either she knew when to turn up because of a message from the Doctor, her future self, or Amy perhaps ... or it's one of those plot points that's going to be handwaved because of the timey-wimey. /sarcasm.