The thing about the Ready action is that it effectively grants you an extra standard action... but it costs you a standard action to do it
So if I move 30 feet (move action), then Ready (standard action) an action to cast a spell if X happens, that's my allotment of actions for the round used up.
But, if X does actually happen, I get an extra standard action to use to cast my spell.
So in the scenario above, I move 30 feet (move action), then Ready (standard action) to mvoe 30 feet if my friend moves 60 feet and stops. That's all my actions for the round. But if my friend does move 60 feet and stop, I get an extra standard action to use to move 30 feet.
-Hyp.