If he bought the fluid at the dealer, he also would have bought the newer-spec 236.14 fluid. MB says it is backwards-compatible, but there are many stories exactly like this one: transmission worked great, put in the new fluid, started shifting hard.

If it is working properly otherwise, I would buy 12L of 236.10 fluid from NAPA (a reliable source), drop the pan, fit a new filter, put the pan back, add 4L of fluid, and then disconnect the cooler return line at the transmission and flush it 1-2L at a time. (There is a procedure detailed in the W210 forum stickies at Benzworld by a guy named yumling.) Of course you also have to buy a dispstick tool, or make one, so that you can get the level set properly.

If while you are in there you want to pull the valve body as suggested above, have at it, but use extreme caution. They are heavy and there are a couple of plastic parts that break if you drop it...and it's really expensive.

Good luck.