It does, after all, seem like a hard line to draw in the sand. On the flip side, it might also impact your relationship with your friend, depending on things like how they broke up , how long they dated, and whether or not they still harbor feelings for this person in question. If the breakup was recent, for example, your friend may have some lingering feelings. They may also feel awkward about situations in which the three of you might hang out after these new relationship lines are drawn.
There are a few different things that happened during the time I was newly separated that caused me to classify myself as temporarily psychotic, one of those being when I figured out "my ex is dating my friend! I seriously felt like I was going to go insane. Here was a woman who I thought was my good girlfriend. We had gotten together a few times and I had told her things -- personal things about my ex and our relationship, why I was getting divorced, etc. I shared things with her and trusted her.