Was Dylan only friends with Eric because..

His great friend that he did everything with, Zack, ditched him for a woman?

I don't think that's the only reason why they became friends. I believe it was Nate who said Eric and Dylan met each other at Ken Caryl middle school and quickly became friends. In their freshman year they formed a group that consisted of Eric, Dylan, Brooks, Nick B, Nate and Zach and their favorite hang out place was the high school library (ironic!)This was from No Easy Answers. Dylan considered Zach his best friend and held him up on higher pedastool than he did with Eric. But their NBK secret made them closer and bond even more.

I believe the shared experience of having to go through a humiliating arrest and diversion program really cemented their friendship. This was what set them apart from their friends who, in the same period, all went and bettered themselves moral and social-wise which probably made E and D feel even more isolated or different and thus drawn tot each other.

I think that Dylan's closeness to Eric was definitely informed by Zach getting into a relationship with Devon, but that it was never the only reason why they were friends. They'd already met back in middle school and struck up a friendship there, so my guess is that this friendship evolved into something stronger over time partially due to the shared experience of arrest/diversion (as MegaLoX already said) and partially due to Eric falling out with most of their shared friends and Dylan being left to his own devices more as Zach scored himself a girlfriend. I really think Dylan was all Eric had left, and Dylan opted for Eric because Eric was always there in turn.