Playdate Issue Please Help

My 6 yr son goes does playdates and sleepover often with his 8 yr old friend who is also a boy.
Its been quite a few months that they are having but, off late my kid mentioned to me that is worrisome. His friend actually tries to smell inside my kids pants when he thinks my kid farts. My kid also mentions that he told the friend every time he does that. Also the friend told my kid that he will not tell this to anyone. I just think 8 yr old and 6 yr olds are still very little and not matured enough. I am not sure whether to discuss this with his parents or whether to stop sending my child. Also this family is very close and my kid still likes to play with the friend. I just feel discussing this shouldn't create misunderstandings. I have explain my child what is safe and what is not safe. Suggestions please welcome.