Best friend’s ex

So I’ve known my best friend since the 5th grade and we’re really close. She dated this guy in 8th grade for about a month and then they broke up. Maybe It’s not a big deal but this was her first “serious” relationship and she was really heartbroken when he broke up with her. We’re seniors now and he’s been dropping hints that he likes me, I kind of think he’s cute and he honestly has a great personality. My friend is the jealous type but she says that she doesn’t have a problem. I know in my heart that there will be problems in the future if me and him try to have a relationship, but what if he’s the one? What should I do?