I think it's nearly impossible for one checked bag to cost €1100. The difference between a "light" fare (no checked bags) and a "standard" fare (1 checked bag) is less than €100 per direction. He's probably comparing different things (a one-way flight vs. a return flight for instance, or economy vs. premium economy or better).
Even if the flight is booked on a light fare, you can add a bag during the booking process (it's quite more expensive than the difference between light and standard, though!) or, once booked, via the "manage my booking" interface on the web or in the app, or in some cases even during the online check-in process. It's also possible at the airport. It will be cheaper if he does so at least 24 hours before departure (so online rather than at the airport).
Whether he can use the "manage my booking" features or not depends on the arrangements with the travel agent. But online check-in is nearly certainly possible.
I tested on a random CDG-LAX flight, the cost is €192/bag during the booking process. It will be more expensive at the airport (can't tell you how much).
Note that frequent flyers with at least Flying Blue Silver status automatically get one additional checked bag for free (sometimes 2).