If your friend's car seat moves from side to side, it isn't installed properly. When you install the car seat with a seat belt, you thread it through and pull the seatbelt out ALL the way and then let it retract. Your friend probably stopped at that step. Next, you need to place your knee with all your body weight into the car seat (the part your child would be sitting in) and then pull ALL the slack out of the seat belt. If you do that, you will get a really tight fit with no movement at all. Thus, I don't think the problem is your friend's car seat, but the manner in which he installed it.
That being said, we love the Britax Marathon. I have three installed in my minivan with the latch system (the lower anchors AND the back tethers). I have another 2 in my 1999 Nissan Pathfinder that I installed using the seat belt only. Important to use one or the other. Don't use latch and seatbelt or you are overtethering.
The only car I've ever had a problem fitting a car seat into is my husband's Audi convertible. That is because you need to remove the headrests when installing a convertible seat (another thing that many forget), but in a convertible, you can't remove the back headrests (or at least in his car you can't). So we only use low backed boosters in that car, which means our 4 year old twins can't ride that one...