There's no way I can imagine Dylan going through with it alone. Had Eric not shown up his name would have been added right next to Zach's on Dylan's list of dissapointing friends who had "abandoned" him. It would have made his depression even worse, but I'm still not sure Dylan would have done anything about it. It would have been an "Oh I knew it was always a joke" in person with Eric, while he continued dreaming of suicide privately. Without Eric's persistence Dylan wouldn't have made it to the "go through with it" phase of his violent and full on self destructive phase of NBK.
I'm not sure that Eric would have continued on his own either though, at least not on 4/20. Remember Dylan helped set the bombs intended to cause half the destruction on NBK. It's possible Eric may have settled for 1 cafeteria bomb, but that would have severely cut back on the body cout. Eric was all show when it came to the attack. He spent months of planning for a body count worth giving his life for, and that body count included Dylan's cooperation. I don't think Dylan wavering would have changed Eric's actions, but I do think it would have caused a few weeks set back. He'd have simply waited another paycheck worth of time and built a second cafeteria bomb on his own.
Dylan was there to help raise the body count for Eric, while Eric was there to facilitate Dylan's suicide. It's hard to say exactly what would have happen if one chickened out due to their vastly different motivations behind their killings.