Yes, 7.5 litres according to the info on Mobil's link, but if you have a handbook check it in there and tell him not to put the full amount in to start with, as over filling will throw up an overfill fault warning.
Don't know if you have the luxury of a dipstick, I don't, mine is electronic, so as mine takes 6.5 litres I added 6 litres, checked it and the reading said 'oil level ok'. I then added another 0.25 of a litre. Always better to be slightly under than over with Mercs.
I found out while doing mine that removing the plastic filter holder is a bit awkward due to lack of access space for fingers/hands. I have now bought a tool/wrench for next time. It may be worth checking that he has one of these as although it's do-able without it's damn awkward. If you intend to order one, make sure to get the correct size. You will need to measure the filter holder and count the number of flutes.
mercedes oil filter wrench - Google Search
I'm assuming that your filter is mounted at the top of the engine at the front, the same as mine.
You should also have 3 different sized O rings with the filter if it's the same.
P.S. If you go with getting the oil from EuroCarParts, the MBClub discount is only valid for online transactions and not available if just popping into one of their shops. Must be ordered online to qualify!