Dating the ex of a friend is sneaky.
Sandra, who used to be married to Dave's best friend, asks Dave out on a date.
Dave wants to respond to Sandra's request for a date.
Dave tells Sandra that his friendship is worth more than sex and declines her offer.
Dave tells Sandra that he's always lusted after her and that he'd be delighted to get serious with her.
Dave's best friend finds out Dave declined and is delighted that he has such a supportive and upstanding friend.
Dave's best friend finds out and is deeply hurt that Dave is dating his ex, who so badly hurt him.