Is this considered cheating

Let’s say hypothetically that a person is in a committed relationship. At work that said person has a coworker that has a crush on them and is flirting with them constantly and the person seems to enjoy the attention and even playfully returns the flirting. But the person in the relationship says it won’t go any further than that. Is that still considered cheating?

I ask this because I went and visited my girlfriend at the restaurant she works at and ate dinner there yesterday. Almost the whole time I saw her and a co worker being way too friendly with each other. I couldn’t even finish my meal and just left