What did he use to check the fluid level? Save for a few very early 2005s, the transmission tube is capped with an "authorized personnel only" warning, and for good reason: The Mercedes transmission mated to the 3.7s is extremely sensitive to fluid level and quality.
If he just dumped some ATF+4 (hopefully it was ATF+4) in there until it felt good, that could be the entire problem.
At this point, I'd recommend towing it, not driving it, to a trusted shop or dealer and letting them sort it out. Dealer's probably your easiest bet, but a well-equipped independent shop may have the tools they need.
2006 WK Laredo 4x4 4.7L
1993 ZJ Laredo 4x4 4.0L — Dead, sold, and rolling again