If you're no longer friends with this toxic person and her and him are no longer seeing each other then go for it. I am dating my ex best friend's ex boyfriend who is also my ex's roommate/best friend. They are no longer friends due to my ex being a completely shitty person. And I am no longer friends with this girl bc she's a selfish person. But I did start talking to my current boyfriend while I was still friends with her but they had ended their relationship 3 years prior to me breaking up with my ex and taking to my current bf. I was really worried at the time about telling my friend bc I didn't want her to feel like I betrayed her etc bc their relationship did not end well since he dumped her. But when I finally asked her if she was cool with it she didn't seem to care at all since she was in a new relationship and it was years ago. Then eventually a couple months after we had a fall out. Honestly dating my bf now is the best choice I made, we've been together for a year now and probably will be moving in together later this year. He's all around so amazing and I find it funny I never had any interest in him prior. Do what your heart tells you! If these people don't care about you then don't care about how your actions will affect them! Do you!