It's obvious from the outside: the person isn't good for our friend, plus being in a relationship isn't mandatory; they can totally do life on their own and avoid unnecessary suffering, but when you are the person in a relationship like this, everything is much more complex.
It's like that metaphor people use of the boiling frogs. Bad things don't happen right from the beginning, and they also don't happen all at once. They usually come mixed with very good moments, nice trips, beautiful gestures, and one is often too deep in the relationship to back out once they start becoming more frequent.