Just learn from your mistake. There are other fish in the sea. Plus, if he's your best friend, confessing to him might put your relationship in an awkward situation. They're both your best friends too, so if their relationship progress, just be supportive. But yes, it's a mature thing to do and it's hard. It depends on you.
Imo, it doesn't matter if she's playing him. It's the guy's choice to entertain her or not. Like I said, if they progress, support them. The same thing when the opposite happens; be there for them too. Or talk to her instead. You know her the best, so if you're really against her flirting with him, I think you should say so.
So i kinda like my best friend the problem is my other best friend knows i like him so she asked me if i can let her hit on him i said yes because im stupid and im kinda regretting so.. any advice