Couple of things:

- No fluid change for 150K miles is not a death sentence. MB initially did not recommend any service for the life of the transmission. Later this was changed to a single service at 40K miles because the gearchanges would become a bit rough. I would not assume the fluid change caused the problem.

- The transmission uses specific fluid. It can be had from the dealer or specialist vendors (e.g. this site). Although it is similar to Dexron, you cannot use common over-the-counter fluids in this transmission.

- The transmission is sensitive to fluid level - aren't they all.

- The transmission dipstick is a service tool, it is not included with the car. The car itself has a more-or-less standard tube and cap, in more or less the usual location, for checking the fluid. Just that there is no trans dipstick attached to the cap. The cap has a replaceable locking clip which is destroyed in the process of removing the cap. This exists to prevent people from adding incorrect transmission fluid types.

- You can buy the trans dipstick here, or make your own. At either this site, or perhaps over at mbworld.org, you can search up a thread on how to make a transmission dipstick at home. Google is your friend here.

- Fluid level is supposed to be set with fluid temperature at 80C. If you fill the trans cold, it will be overfull.

- Given the symptoms, I'd be checking the trans fluid level before anything. If it's low, I'd be looking for where he poured in the 3 liters of fluid...