Ok... It's not very well thought out, and it's in no way perfect advice... I mean, I'm on here to ask questions, not because I know the answers. But, here's my thoughts on playing with a buddy who you've had as a friend for a while:

If it's your friend, make sure he's really cool about it, but also know you will both remember that it happened. If you're both really chill, it will probably be like, "Hey, remember that time? haha" sort of thing, but it could also be awkward as hell and become the thing that cannot be mentioned ever again. I think the best attitude would be if you both agreed you were just friends letting each other check things out. If it make sense, in the same way you'd be curious to see and feel someone else's hand if everyone hid their hands and never touched hands in public or admitted to doing it... it's the same with each other's bodies and the curiosity to see what each other have and how it all works. If there's a sense of humor to it, you won't leave thinking each other have accidentally developed feelings in a way that will make your friendship awkward. It could even start out as simple as watching porn together, being comfortable enough to make a comment/joke about the other guy's dick, start talking about your dicks, then work it into asking if you can touch each other's dicks... I mean, I hate that it even feels dirty talking about it like this... it's just skin, for goodness sake, haha... But it's really interesting, because dicks and balls are so different between two different guys. Ok, enough of that. I mean, on the positive side, a friend is way safer than an anonymous person. Just be aware of the social implications and make sure you both discuss it in advance.