When i first saw this thread, it amazed me that there's 7 replies to this post. When i read those 7 replies, i was even more blown away that there's so many different answers.

You can run sram chains on shimano cassettes, with no worries. Some folks report shifting finickiness, but i've run sram chains on shimano 8-, 9-, and 10- speed drivetrains without any discernible issues.

your mechanic is either a fool or a liar, and you need to find another mech or a toolset for yourself.

-rob