Should I tell a girl her boyfriend is cheating?!

Tell her

She'll find out sooner or later probably, better it was sooner.

...If so then why tell her.

Because, as I said, it's better she found out now rather than a year down the line, or if they get engaged or married or who knows what could happen.

Even if she wasn't going to find out, she should still know so she can choose for herself

Backstory- Dated a guy for 3 weeks in Feb '16, I ended it as he was really flaky. We hooked up again in May and he told me after he'd had sex with my twice that he was dating a girl but it was early days and they weren't exclusive. I was simply after a FWB and casual situation so thought nothing of it. That was until he told me to use code words when I was texting him Incase she was around and saw them. I realised this wasn't someone he was seeing 'not exclusively' , confronted him and he admitted they were together and she had a low sex drive and wasn't exciting in bed, hence why he looked elsewhere.

I asked if she was aware of this and she wasn't, he was cheating. I ended it there and then. Felt no love lost and wasn't exactly heartbroken just felt I had a lucky escape! I strongly urged him at this point to not lead this girl on and be honest with her. Fast forward to September and I'm now dating an amazing guy and really happy. I receive a text off this guy and start wondering why he's speaking to me...he was wondering if I was up for a 'quickie'. Informed him politely I'm happy and asked if he was still with this girl, turns out he is and was looking round for a bit of fun.

*I've been cheated on before and my heart truly goes out for this girl. She seems really into him on social media yet he's messaging me and probably other girls. I'm also concerned for her safety. When with me he stated he wasn't sleeping with anyone else and we didn't use condoms, I got an STD test immediately after I found out that wasn't the case. Fortunately it came back clean for me.

*I'm in a dilemma. As a fellow female and woman, also as someone who has had a boyfriend cheating on them...it's heartbreaking and I wanted to know. Despite it being hard to hear, I came out better the other end. I want to show her the screenshots and let her decide what she'd like to do with them. I have no emotions or feelings for this guy at all, he's not a friend of mine nor do I like or dislike him. I just know what he is doing is wrong.

*On the other hand- is it my buisness? I'm happy in a relationship of my own, why is it my buisness to meddle in their affairs?....what should I do?!

??? really no one's business

It's the girl's business and she deserves to know

you know what I mean. It's ot YOUR business. Tryna be a do-gooder while interfering with the dynamics of someone else's relationship? Wow. If you wanna be a do gooder then donate to a children's charity.

I personally don't get involved in other people's business unless it's someone close to me. Up to you and your conscience though, it's only for you to decide.

Who **** in your cornflakes this morning mate?

If you mean me personally, the OP asked a question, I gave my opinion, hence the point of the thread/tsr, which most people seem to agree with.

If you mean "you" as in the person in the OPs position then that is your opinion and I understand it. However, I would want to know if my girlfriend cheated on me. She has a right to know if she is in a relationship with a person who doesn't respect her enough to remain faithful to her. If she doesn't care then that's fine, if she does she'll be able to save herself from a bad relationship getting worse and worse the more it progresses.

The guy didn't even make a "mistake". He has been actively seeking out sexual partners for months on end and she should know that.

Why did you quote mistake

Because it's debatable if people who cheat once if drunk or whatever is really a mistake or not, and it's usually the excuse people give for it. However in this case it clearly wasn't just a mistake and has been actively searching for people for months.

That the only counter argument you can make to my post?

Okay