Hi Long,
In my opinion, you have a difficult delima here. Your friend is definitely bi (gay)-curious, and is throwing out bait for you. It's the ultimate fantasy for many guys.
I don't think I recall that you said anything about whether or not you are attracted to him, but I'm assuming that you are. We all know what your cock is hoping will happen, but make sure you get your brain and heart into this decision also. As long as he is married, it is cheating, both on his part, and yours, because you know he is. This coupled with the fact that you have been best friends for so long, makes this situation very risky in many aspects, most of which have been presented in previous posts. You also don't know how he is going to react, or at least feel if you two have any type of sex play.
I hope that you understand, I am not judging you at all. It's something that I have battled with for years, but on the other side.
Good Luck