If they are taken its best to let them decide what they want. Let them end their current relationship. Cause cheating never works out well. I've been in that situation, as the person needing to make a choice. So far I've chosen the person I've always been with. And its not that simple of a choice. If you want to wait for this person that might be a good idea, but I wouldn't wait too long. Because they may never leave the person they are with.