You might find some useful information on here
http://www.beaumontsociety.org.uk/
If it gets to the point you are really uncomfortable though, I would suggest you take a step back from the friendship and allow him the freedom to be who he has wanted to be for years. He appears to have restricted his lifestyle for the sake of his wife, he is now in a situation where he can be himself - it wouldn't be fair for you to appear to be supportive now, only to place further restrictions on him or resent him further down the line. You do sound like a good friend though, and I wish both of you well - enjoy your trip! x