Good friends are always comfortable with one another to talk about anything that needs to be talked about. If you can do that, you have no secrets. Having no secrets, to be comfortable in your own skin, to acknowledge that the bits between your legs are normal, natural, and nakedness is nothing to cause shame.
Yesterday, a good buddy of mine was comfortable enough to tell me he's having issues. He wakes up with good solid morning wood - but the minute he wraps his hand around it, it wilts.
"In this case," I told him, "it's all in your mind. Though, you've got a really nice cock," [yep, I've seen it, plenty of times; even with a semi-boner] "Your brain is the biggest sex organ you have."
He needs to relax and let his mind wander - drop himself as deeply as possible into a sexual fantasy, play with his cock and balls, and concentrate on the sensations.
"If that doesn't work, let me know. I'll be happy to take a stab at it for you." We laughed, of course. But, I'll make a serious offer if it continues to be a problem for him. I mean, isn't that was good friends do?