Again, it was Harry himself who was calling for help.
For me, what makes Peter seem dickish here is his attitude about the whole thing. "Harry brought this upon himself, and there isn't much I can do to help him since he won't get off the drugs. Meanwhile, Gwen Stacy's murderer is still on the loose. I can't stop Harry from doing LSD but I CAN stop the Green Goblin from killing again" would sound reasonable.
But he phrased it more like, "Should I stay here to help my whiny little bitch of a friend, or should I go kill the guy who wronged me? Hmm... killing it is! Ha, ha! As if mere friendship could stop me from getting my revenge on!"
But, then again, Peter was going through a terrible time just then so I guess one can excuse him for being so overwrought about it all.