Okay, no. It wasn't clear Mark had a romantic interest in her. It would be fucked up to assume that his intention behind being kind to her was to get with her. Ross thought that was the case because he had his own insecurities, and even though it did end up being true, Rachel did nothing to hurt the trust between them. She even said she liked spending time with Mark because he was in the same field of work as she was, she had no romantic feelings for him, and even turned him down later when she was single. It's not her fault.

After a fight with her boyfriend, she needs some emotional support, just like most normal human beings. Her friend comes over in what she saw as a friendly shoulder to cry on. It's not her fault that this guy had other intentions. Her boyfriend can't even support her career, and you expect her to somehow manage his shitstorm of emotions on top of her own?

She didn't break up with him, either. Obviously that's the joke of the show, but like... "let's take a break" "let's break up". Pretty specific choice of words. Regardless, who in their right mind gets out of a long-term relationship and then immediately has sex with someone else to spite their ex partner? How is that not totally out of line?