I don't sit on the sideline when my friend is having a hard time, but I also don't assume I know their relationship better than they do. For example my best friend was in a shitty relationship a couple of years ago and when he told me about what was going on with his boyfriend I was like "bestie, you deserve better, that behaviour is not OK, what do you want to do". And he was like "I kinda want to break up with him but I'm anxious" and I said "I think you should do it, how can we make it easier for you?"

But he ultimately didn't go through with breaking up at that time, and a few weeks later the BF broke up with him instead.

In that time I shared ideas with my friend re the pros and cons of why his boyfriend was acting like a piece of shit - excuses being depression, lockdowns, long distance - and less valid reasons like being an arrogant entitled person who is bad at expressing emotions and takes my friend for granted.

What I didn't do however was go "I know why your boyfriend is acting this way better than you do". And I also didn't write an article about how my friend is toxic for feeling the way he felt.

There's a difference between showing support, giving advice when needed, allowing your friend to vent - and meddling in their affairs.

As for toxic - the issue for me is that according to what you wrote, your friend is mostly feeling betrayed because his girlfriend KISSED this other guy, not just because she "looked at other people with admiration".

The trust was broken because of her ACTION. Not her emotions.

It's not healthy to expect someone to never have feelings towards others while in a monogamous relationship, I agree with you on that.

But it's entirely reasonable to expect them to not ACT on those feelings. That's the trust.