Problem with seatbelt install in a 2005 Tahoe

Today I helped a friend/co-worker install the FR85 that I sold her. Prior to buying the FR, her son was using an EFTA that she has now passed on to her 8 month old. She moved this seat to the second row behind the driver side and has to install with the seat belt because there are no LATCH anchors on that side, just in the center and the passenger side. I checked her install, and there was a bit of movement still, so I showed her how to push down on the seat while feeding the belt back into the retractor and making sure the belt was locked. After that, the seat was in nice and tight. She then mentioned that she has noticed that the seat belt will loosen on its own and she's had to reinstall the seat! I told her that is not something that should be happening and that she needs to have it checked out. Has anyone else heard of this happening?

I would have advised her to put the baby in the center but I don't know if it would fit next to the FR85 plus that wasn't an option because her 3 year old isn't always nice to the baby! So for now, I told her to check the seat every time before she puts the baby in it until she can have it looked at.

Double-check that she is in fact locking the belt correctly. Besides that, the only thing I can think of is that the carseat is shifting on the vehicle seat with repeated use and altering the install. It may slipping and losing recline, which would add slack to the belt. Most likely IMO, since I have had this happen with DH's truck and a RF seat, is that the locked shoulder belt is pulling the seat up on the side and shifting it around. The low connection point of the lapbelt on the fixed side seemed to aggravate this. I solved that issue in the truck by using the locking clip to install rather than locking the belt. Hope that helps.