i’ve had this issue in the past. It’s definitely the coding of the seatbelts.
SOMETIMES it helps to repeatedly tap whatever button they use for seatbelts (as the code somehow was thrown out of sync) until they’re able to get out. They should then be able to get back in and out of the vehicle just fine.
Other times, I have seen that a player is stuck mid-emote which can cause them to get stuck. So have them press whatever emote canceling button you have designated in your city (or through their personal keybinds) in order to set themselves free.
On the occasion you may have to deep dive into the vehicle as I’ve also seen some vehicles to just scuff. This would for me the impound their cars, and they pull it from the impound which typically fixes it quickly before having to check any of the meta codes. I hope this helps.