They need to start drilling the message into their son that HE IS AMAZING and PERFECT just the way he is. That they love him and are proud of him and who he is.
Also, at 6 kids very often mimick what they see. My daughter has a little boy friend (both are 5) who has a "princess walk" that he modelled after my daughter, where he sways his hips and does a little wave. He does it because he thinks it's funny, not because he's gay. I mean, he *may* be, but that's not WHY he's doing this.
Tell your friend and her husband to put what other kids will do out of their minds. Their son will pick up on their worry and concern and start to think there IS something wrong with him. They just need to love and support and encourage him; he needs a lot of self confidence to deal with some of the people we share our world with - and this goes for ALL kids, not just the 'different' ones :)