Big big rant about a first time in a situationship. I promise it gets interesting!!

Hello guys! It's the first time I am writing in reddit for something like that, so please be gentle with me and don't hate me for the huge rant!! Haha 😅 I (36M) had a thing with a girl (31F) last year, for about 3 months. Fact is that I didn't know what a situationship is, until I lived it. So I met this very cute girl in a bar, talked to her, exchanged numbers and started texting about when to set up a date. After a few dates, which were perfect, from drinks to theaters and restaurants, we eventually got intimate and had sex. Which was also perfect, every single time, except from maybe the first time, which was somewhat a journey of one discovering another's body. Sex was so good for both, that she even mentioned the fact that after many years she was finding herself enjoying having sex, because in her last big relationship (which was about 3 years) she didn't enjoy sex at all, and that's when she promised to herself that sex would play a big role in her future relationships. She worked evening hours, so about midnight I would go to her house 2 times per week or so, spend the night there (also asking her if she would like me to leave after sex, in order to sleep alone or whatever.. I didn't wanna apply pressure to her, but every time she wanted me there) and then the next day go out for breakfast, coffee and generally stay with her until next day 16:00 when she would have to work. We told each other everything about our lives, and it was obvious that we had a very good chemistry. We continued meeting often, indoors or outdoors. It felt like it was progressively going into something beautiful, and I liked it. I started having feelings about her because we did all the things that couples do. Waking up to find messages and reels from her was such a pleasant surprise for me, because I wasn't used to a woman showing such interest to me. Usually it was the other way round. I also received late night calls from her at 03:00, ranting about her past mistakes, the pressure that she felt about her new job and many more.. I was understanding and tried to help her with those issues. Fact is that maybe I shouldn't have been so understanding and good with her.. Because when we reached 20th of December, we had our last meeting before Christmas break. Again visited her after work, spent the night together, next day went for brunch and shopping, and then kissing each other goodbye for the Christmas holidays which we would spend in different cities with our families and telling each other that we would meet in about 8-10 days. So.. During the holidays, she was getting colder in the messages, I could feel that. Slow and taciturn replies, the exact opposite from what I was used to. And at the 30th of December, she texts me this "I wanna share something that bothers me with you. Unfortunately, there's no right timing to say something like that. I had a great time with you, but for me it's obvious that we have to stop seeing each other. I can't do this anymore. I am so sorry.." Needless to say that she fucked my feelings. I never saw it coming, because she had expressed thoughts and feelings that you normally don't share with someone that you plan to just have sex with and then dump him. I tried to reply in a calm and polite way, and we stopped talking after a few texts, because it was obvious that she had made her decision. 10 days later, she wishes me for my name day, around midnight, telling me that she also lost her grandma and she had a bad day. Again, I replied in a polite and respectful way, telling her to reach out to me if she needs something and we never talked again. From what I can tell, she was obviously weak when we first met, then gradually felt by as the time went by, and when she felt really really good with herself, after healing during the time she was with me, she dumped me. NOW, let's go to the REALLY interesting stuff. I found out that she's in a relationship with a guy since we broke up. From what I can tell, she must have started dating him while we where together, because very shortly after we broke up, they were together. Fact is, about a month ago, that this guy is following ANOTHER girl and starting texting each other, while being in a relationship with my ex. Liking her stories, photos, and subtly kind of flirting with her. As a man, you NEVER like photos and stories of a girl you don't even know, when you have a relationship. Turns out this girl is a friend of mine and I accidentally found out about him texting her when she received a message from him while we were having a coffee and she showed his profile to me. Now, what do I do? My friend asked me if I want her to follow my ex on Instagram, so that the ex will see the mutual friend (aka the guy) and start having doubts and questions about him, and she even proposed to go one step further, texting my ex out of the blue and asking her if she knows the mutual friend (aka the guy) because they have been chatting for a while and she wants to know more about him from someone who knows him, since he's a complete stranger. What do you suggest? Do I stay put and tell the friend to simply stop talking to him, or do I tell her to go all the way, follow her and talk to her about him? I know it's bad for me to say it, but part of me wants a revenge, wants my ex to find out about him talking to other girls.