The first thing that you need to take into consideration is how long it has been since your best friend and their ex broke up. If the relationship ended recently, they may still have strong feelings for each other. Making the decision to enter into a relationship with the ex at this point in time should be avoided at all costs. You need to give both people time to get over their feelings for each other and to get past the breakup.
If you are seriously considering dating your best friends ex, it is a good idea to talk to your friend about this. There is nothing worse than feeling that your best friend went behind your back and started a relationship with your ex without even having the decency to discuss it with you. Spend some time talking with your friend.
Let them know that you are interested in a possible relationship with their ex. Find out what their honest feelings are about this. While it is not guaranteed that they will be honest and tell you exactly how they feel, you can tell a lot from body language, tone of voice, and what they say. Pay close attention to these signs and clues. If they say that they are fine with it, but their body language and all the signs tell you differently, you need to proceed with extreme caution.
Think about how this relationship will affect everyone involved. If you and your best friend usually do things on double dates or things like that, this will be hard to do when you are dating their ex. Think about how you are going to handle these situations.
How much of an impact is this decision going to have on your relationship with your best friend? Are you going to have to completely rearrange your life and what you do? It is important to have the answers to these questions and know what you are going to do before you start the relationship.
If your best friend has major issues with you dating their ex, you are going to have to make a major decision. If you start this romantic relationship, it could potentially ruin your friendship completely. You have to decide if the possibility of a relationship with someone that interests you is worth losing your best friend over.
If you are that sure about wanting to date the person, then perhaps it is worth the risk. But, it is going to take a lot of serious thinking and decision making on your part. Whatever you do, make sure that you are upfront and honest with everyone that is involved. This is perhaps the best way to prevent problems and animosity.