Let your best friend know that her crush confessed to liking you. If she has an issue with it, you have to make a decision on whether it would be worth it to pursue a relationship with your best friend’s crush at the risk of possibly losing your best friend.
If you don’t deem it worthy, you should let your best friend’s crush know that you aren’t interested in him. There are a lot of guys out there that can be romantic partners to you. There aren’t as many people who have the potential to be your best friend.