I had that happen w/ a friend a couple of years ago. We weren't certain what to do, so we played two. Afterward we looked up the rule, and it is quite interesting. Basically, if the person has a play on the ball after it hits the resting ball, play continues. But what what is really interesting is if the player believes he/she is hitting the correct ball, it matters not which ball is played.
In other words, mikeler hits ball a and it hits the resting ball, ball b on my court. If at the point of impact it is not clear which is a and which is b, and I play ball b, then play continues.
In either case, if neither ball is playable, mikeler wins the point.