Personally, I detest the whole idea of stag and hen nights, what they are meant to symbolise, the activities they entail and the mahoosive costs when folks seem bent on outdoing all their friends. They always seem to result in some kind of argument or upset - someone feeling pressured to fork out what they can't afford to avoid offending a "friend," arguments between attendees when tanked up, injuries resulting from "pranks," bullying within the group of "friends," people concerned about what their partners will get up to, the lot. I seriously don't understand why folks proceed sheep-like into having them, knowing that there will almost always be grief of some kind.
So there, that's my view.
In your friend's case, there seem to be two issues here that are linked.
One, she's not happy about her partner going to a strip club for his stag night. That is not unreasonable for a whole range of reasons from not wanting her DP to invest in the exploitation of women to the fact that whether by choice or coercion, the "stag" could end up engaging in sex with another woman or women. Few people I know object to strip clubs because they are prudes or dislike sex. They dislike sexual exploitation and the breaches of trust that strip clubs rely on for their survival.
Two, she's uneasy about broaching the subject with her DP. Regardless of what the issue is, if there is something that makes a one person in a relationship feel uncomfortable or unhappy and they don't feel they can or have the "permission" to raise it, in my mind, it's a problem. I'm not sure from the OP whether it's the friend who feels she can't say anything for fear of being labelled a prude or whether she feels she can't say anything because her DP will not welcome her opinion, value her views or perhaps be hostile in his response.
If it's the former, she really, really needs to summon the courage to say what she thinks. Her DP may not know that his idea of the strip club is so hurtful to her. It may be that he doesn't actually want to go but his "mates" are dragging him sheep like into doing what you are "supposed" to do on a stag night. She needs to be honest with herself and with him.
If it's the latter, I would wonder if there are other subjects where he insists it's "my way or the high way," and the friend may need to be thinking long and hard whether she wants to spend the rest of her life with a person who feels entitled to exert his views, his will on his partner.