Just to clear yourself of any liability, write the following letter to the former developer, and post a copy publically on the project page.

Dear [open source guy who abandoned a project]:

I've looked with great interest at [project]. The sourceforge page shows that the project activity is nearly zero, and the last software release was done almost three years ago.

It seems like such a waste to have the project stagnate, so I wanted to take this opportunity to pass along a few comments. Namely, I want to inform you that I will be resuming development of this fantastic application. You've done a fine job, and it will be a great honor to continue in your footsteps. I also have a few other things, I feel you should know.

I don't know how to put this delicately, but: YOU LAZY FUCK, YOUR SHIT IS MY SHIT NOW. It's TOO FUCKING BAD you can't finish a single fucking thing that you started, you ass jockey sloth. NEXT TIME, WHY DON'T YOU MAN THE FUCK UP. Take a little responsiblity for ONCE IN YOUR LIFE YOU WORTHLESS SACK OF PUBES.

LAZY FUCKS like you probably don't even finish taking a shit. WHAT THE FUCK IS YOUR FUCKING PROBLEM? YOU DISAPPEAR LIKE SOME FUCKING FBI SUPA DOUBLE TOP SECRET AGENT. You are a failed open source developer. WHO THE FUCK ARE YOU RUNNING FROM? The honest answer, as a failed open source developer, is your fucking shame!

Please feel free to send me any comments or advice. I look forward to hearing from you.

Warm regards,
PMF