Why did Eric tolerate Robyn but not Devon?

I think you meant to say that Devon couldn’t tolerate Eric. lol  (She ended up reporting him to the office at one point because he threatened her life).  Let’s not forget that Devon was Zack’s girlfriend and Eric was resentful of Devon for being like the ‘Yoko Ono” (her words) that stole Zack away from their little Three Musketeers broship. So, he saw her as a threat. Robyn was no perceived threat to any of their relationships as she was simply Dylan’s close friend and Eric’s too by association to his best friend.  Had Robyn taking up with Dylan in a relationship you can bet it would’ve rubbed Eric the wrong way.