MSPeconomist

As he goes toward the main part of the terminal in CDG (following connections/transit signs, probably involving an very short ride on an underground train), he needs to look for the SkyPriority or Aces No. 1 line, IME usually toward the far left as he approaches a lot of passport control (immigration) desks shortly after leaving the little train. He should not get into the main line with coach passengers. There is likely to be an employee checking that people are entitled to use this special line, so he needs to show his MSP-CDG boarding pass, which should say SkyPriority in the upper left, assuming that it's either printed from the DL website (OLCI) or by a DL agent at LAX or MSP. The AF lounge would be after he's done passport control and is close to the gate for the ATH flight. (At LAX, he should also look for SkyPriority signs for check in, security--especially if he doesn't have CLEAR and/or PreCheck--and boarding, and similarly when boarding the flight in MSP.)

He has lounge access at LAX, MSP and CDG even if there aren't any problems. The LAX and MSP lounges (use the one at MSP between F and G as it's closer to the international gates and is much more likely to make announcements about potential delays for the flight, etc.) both have bartenders, so he can enter at 18 rather than 21 (but not drink alcohol of course). In France, there's no concept of a 21 drinking age, so lounge access (with self serve wine and beer) is no problem. I'm not sure whether AF will serve him on the TATL flight, but they will on the intraEU flight, although AF generally doesn't have alcohol available on morning flights.

I forget whether there's a security check at CDG T2 when connecting from a (DL or AF) USA flight to an AF Schengen flight, but if so, he needs to look for the SkyPriority/Accces No.1 signage to save time; there's no equivalent of PreCheck or CLEAR in Europe in general.