.ipa hack help

My favorite free ipod app has updated and is now a complete disappointment. unfortunately, i've dropped the ball and emptied my trash before knowing that I could've dug the old version out of my trash and re-installed it. Since then, I've found someone else's copy of the old version of the app online, but I would need their apple id password to install it. I unzipped and tried editing the plist to enter in my own apple id to get this old version on my ipod touch, but when I try to install either version of the old app (the one with my apple id and the one with his), itunes syncs, backs up, etc and then just ignores the installation request (or it simply fails).

It's important enough for me to get this app back that I've had troubles letting this mission go. the app was and still is free, so there's no theft involved, and my correspondence with the app creators has been completely ignored, so I've tried "the proper channels" first.

This ipod was provided by my job, so jailbreaking is not an option.

Any suggestions?