As a mechanic, it is frustrating for other people to work on something and then give it to me to fix, then I fix it, then they fix it some more, then blame me for it not working right. It doesn't matter how specific I am or how I try to cover my ass, it will sometimes come back on me. I quit fixing stuff for people. Your partner at work sounds like a gem. Hopefully you can steer clear from him. It sounds like your "helping" him wasn't helping him. You may have just been doing his work for him, which doesn't help him or you. There are many people who will take advantage of others when they can. Even if your manage knows what happened, it probably still looks bad for you.
If it was me, I'd kick him in the hoohahs. I have no patience for that stuff.
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Things aren't always the way they seem-- At least that's the way it seems