Stag do dilemma!

I would tell your friend what they are planning to do, she may be able to intervene and explain to them that she doesn't want them taking him to a strip-club and that if they want to go then they should go alone. I personally would be extremely offended if my H2B was made to go to a strip-club or was bought a strip-o-gram and if a friend knew this was going to happen I would honestly hope that they would tell me.

I'd be a bit cautious tbh - are you absolutely sure that the stag doesn't fancy it? I'd just say to your friend, do you know what OH's doing for his stag? and then that kind of prompts her to get involved.

That's a toughie. I think I'd leave it to be honest - you don't know if your friend has already had the discussion with her fiance and come round to it or whether her fiance has already said he won't go to a strip club or if they're both happy with the "what happens on the stag, stays on the stag" route. If you're really worried maybe try being a bit casual about it with her, along the lines of "so, do you have any idea what X is doing for his stag then?". If the answer is yes, you can safely leave it, if the answer is no then you can maybe sound her out about strip clubs without revealing that you know what the BM has planned eg "wow are you worried? What if it's a strip club or something" and go from there.

At the same time could your H perhaps suggest gently to the BM that it might not be the stag's cup of tea, either using the strip clubs as the reason or the fact that other's are going to split off eg "but rather than split the group up surely X would rather just come to the casino with us"

In the end it her OH, his stag and it's really up to them to sort out between themselves what's acceptable and what's not. If she does end up offended and ask you if you knew in advance, you can legitimately say "well I did hear that was planned but I wasn't sure if it would still go ahead since half the stag party decided they weren't up for it".