I have never seen this happen, ever, where doing the “right” thing-(which is telling the friend the truth)-doesn’t end up destroying the friendship in some capacity. Regardless if the couple stays together or not, YOU will be a constant reminder of the ugly lie that existed between them. If you decide to tell, you have to decide whether or not you are going to be comfortable with whatever the outcome is.
I was once put in a similar situation. I was best friends with a girl, who at the time was pregnant. I was out one night at a bar, and her boyfriend walked in- WITH ANOTHER GIRL. He knew I saw him, and he avoided me the entire night. I decided not to tell her because I knew she would not believe me and our friendship would be destroyed. They had a very dysfunctional relationship, she was very desperate to be with him. So I kept my mouth shut. You know what happened? He assumed I would tell , so he went to her first in an effort to tell HIS story, which was that he was not at the bar with another girl, that he was merely standing next to her and I misread the situation and was trying to break them up. Sounds nuts, right? Well she believed him! She called me to try to convince me that I didn’t see what I saw. When I wouldn’t agree with this crazy story, and instead stood by my story, she couldn’t handle it. She knew I was telling the truth but she wanted me to go along with the lie. I couldn’t do that. At that point, I realized she was so sick, that she had bigger problems that whether or not I saw her boyfriend out at the bar with another girl. She married the guy, our friendship was over, and we never spoke again.
Tina knows that Doug is a pig and a liar. Coming home from strip clubs in the past with hickeys, feeling that she needs to follow him to the bar to “check up on him.” Women who have trust in their relationships don’t need to follow their men to bars to check up on them.
I know you said your boyfriend is a nice guy, but honestly, I don’t think it’s very nice to A) put the burden of a story like that on you and expect you not to tell, and B) to have actively assisted Doug in doing this to Tina. You said they were going to take him to the strip club anyway, even without her permission. Why did they bother to ask? So they could use her reluctant “permission” as way to justify anything that may have happened there?
I feel so sorry for you that you are going through this, it’s so unfair. I think that Doug, your boyfriend, and their friends have a lot of growing up to do. They all sound pretty bad to me.
If they would do this to Tina, aren’t you wondering if they’d do it to you?
ETA: I have different friends today, and my current friendships are as strong as that of sisters. Because our friendship is strong, I would tell them if I saw their husbands cheating. Even if they were initially angry with me, I know our friendship would be able to survive the outcome. (I obviously did not feel that way about the strength of my friendship with the pregnant girl, hence my reason for not wanting to tell her- intially).