If your friend's ex has integrated into your social circle, you may have already formed a connection with this person. Friendships, as everyone knows, often lead to even more rewarding romantic partnerships.
You know him
You know his/her friends if your friend groups converged. You know who to impress to win over his friends. When you meet your partner's friends, there's no awkward small talk.
you already know his friends
From your friend, you may have heard many stories about this person. You know his/her behaviour, pros and cons. So choose someone you know so there are no surprises.
Their good and bad sides are known
Even if you've moved on, seeing your ex may not be fun. If your friend puts your happiness before her/his and approves of you dating an ex, you're BFFs.
Friendships can be tested
You might feel judged if you share too much about your relationship with a friend. Even if your friend says that you dating her/his ex didn't hurt, you may still feel betrayed and angry.
You could ruin your friendship
You may have liked or disliked your friend's relationship. Identifying this person could be difficult. Your friend's ex might surprise you. You may show someone a new side.
You may have ex-related misconceptions
Depending on how close your friend group is, this decision may also affect your other friends. If your friend doesn't like the news, she/he may sway your other friends.
You might be compared to his friend. He may compare you to a friend. By knowing this person's previous relationship, you may compare yourself to them.