I need advice.......PLEASE!!

I'm not sure what to do. I've been jamming with a guitar player for almost 2 years, and the guy vanishes. I've tried to get in touch with him several times over several months, but the dude never responds. I think he fired himself and just never said so. Here's my problem:
It was MY project. He joined ME to collaborate on MY stuff...... BUT......he did help with some of his own ideas and a few riffs are of his creation. He wan't just a guitar monkey. Now, I am moving on without him 'cause I'm not gettin any younger, but what is the RIGHT thing to do? Is it OK to keep those songs as they are, seeing as how it was my project anyway, and give him written credit on the cd credit list? Or should I delete the riffs he brought and re-write something to fill the gaps?? I just don't want to be a dick and have this guy go "THAT"S MY SONG!!" when I'm playing some gig and he happens to be there.....but I also don't want to back track into older stuff and delay what I am currently working on......
What do you guys do if some dude contributes shit and you use it, then that dude quits?
This is tough.......any thoughts????
It was MY project. He joined ME to collaborate on MY stuff...... BUT......he did help with some of his own ideas and a few riffs are of his creation. He wan't just a guitar monkey. Now, I am moving on without him 'cause I'm not gettin any younger, but what is the RIGHT thing to do? Is it OK to keep those songs as they are, seeing as how it was my project anyway, and give him written credit on the cd credit list? Or should I delete the riffs he brought and re-write something to fill the gaps?? I just don't want to be a dick and have this guy go "THAT"S MY SONG!!" when I'm playing some gig and he happens to be there.....but I also don't want to back track into older stuff and delay what I am currently working on......
What do you guys do if some dude contributes shit and you use it, then that dude quits?
This is tough.......any thoughts????
