Having a number of Harley Davidson posters in my own Garage Gallery, here's what I suggest:
1. Vacuum wet mount to Gatorfoam
2. Laminate with PVC film - I like linen finish
3. Frame without mats or glass.
4. Have him buy another poster with the money he saves, so you can mount it, to replace the first one after five years or whenever it gets ratty.
Or, if mats are specified, then use glass with a deep spacer. Also, fill the frame and put a good, tight dustcover on the back. In a garage, temperatures change drastically and often, so the more insulation value you can build into it, the better. Use WallBuddies and BumpOns at least 1/4" thick, to create an air gap for circulation behind the frame.
Acrylic is half the weight of glass and 20 times more shatter-resistant, but it can be scratched more easily, it costs more, and some of the airborne chemicals in the garage might damage it over time. Glass is more stable and easier to keep clean if it doesn't get broken.