When braking quickly or when the truck hits a bump in the road while braking, the seat belt will automatically lock, but will release shortly thereafter. If the seat belt remains locked so that you can't lean forward while driving down the road under normal conditions, then something is wrong with it. That applies to either the driver or passenger side.

 

When she puts the seat belt on does she pull it way out and then clip it in? I haven't noticed it in my truck (because I never sit in the passengers seat) but I was riding in my in a Suburban the other day and it was doing that same thing. It would do it if I leaned way forward or pulled the seat belt out while seated. I heard it was a child seat safety thing. I don't know why anyone would put a child seat in the front seat of a Suburban but that is what they said!