Run away quickly. Someone who cheats has terrible character. They are selfish and unloving and immature. They should remain single until they find someone they love and cherish. They likely had an influence (perhaps from their parents) that cheating was acceptable, but it is not.
Cheating causes pain and anguish, someone who loves you would never even think of putting you through misery.
Someone who cheats once, does not respect you and took the chance on your relationship.