Is OTHER wrong in the post ? POST : I got this friend, lets call her Cindy, that I know since elemntary school. We are both in our late 20ies now. She is now married to «John» and they have a son.

Her family moved away so we lost touch. When I was 18 y/o, I've made some new friends and there was also John. One day John told us, that he met a girl and that they were dating.. Turned out to be Cindy. Fast forward, they are now married since 8 years and having a son.
As life would have it after a few years of marriage, everyday life took over and they wanted to spice up their sex life. So they tried swinger clubs (I know this, because Cindy shared the information with me). Not really my thing, but whatever floats your boat. Right?! In the case of John and Cindy I had my doubts. Both are kind of jealous people. However, they tried it a few times and seemed to like it. To have some security/controll they set one rule - The partner is always there, we do not swing separately.

So last weekend, we were having a girls night out due to her birthday. The evening started very well until she got drunk. We were having fun until I wanted to get some fresh air. Outside she told me, that she is having an affair with one of the guys from the swinger club. She showed me the whatsapp messages and deleted them in front of me. According to her, John is getting suspicious and is checking her phone. I told her, that this is not fair and since she does it behind John‘s back, this is cheating. This was ignored by her. Later we helped a girl that was getting harrased by a stranger. The girl and Cindy started to dance to make the guy disappear. As a thank you she wanted to buy us drinks. I refused and went to the toilett. Meanwhile Cindy and the girl went to the bar to drink a few shots. When I returned, the bartender which is a friend of mine told me that they were making out. I confronted Cindy but she just shrugged her shoulders. I told her that I wanted to leave and that we should get our jackets. She agreed and we went to get our jackets. Somehow on the way to the wardrobe, she managed to disappear. I found her in a toilet cabin with the other girl. It was clearly Cindy. They were clearly making out.

On our way home she asked me if she should tell John what happened. I said YES within a second. She then tried to make some lame excuses. She said something like "I am just not attracted to John anymore / I do not know if I still love him." When I asked why she wouldn't leave him, she said that she does not want to be alone with two kids. She also admitted, she does not feel guilty about making out with the girl. Should I tell John what I know? I gave her time to tell him but she hasn’t until now.

TL;DR: My friend cheated on her husband/babyfather with a girl, in front of me. In addition she admitted that she is having an affair with another man and deleted messages from this man in front of me. They visitied swinger clubs-only rule was, partner is there too. She has not told him yet. Should I tell him?