My response:
Okay, then the truth of the matter is that she DIDN'T have "full coverage" - - she did, in fact, have EXCLUSIONS on her policy, one of which was "other drivers."
Speaking only about the auto damages, and not her liability to the rear-ended party, she has 4 alternatives - -
1. Fix and pay for the damages to her truck herself.
2. Have the boyfriend pay for the damages.
3. Sue the boyfriend for the damages, or,
4. Forget about it, and leave the truck in its damaged condition.
If the other party makes any sort of claim, whether for property damages and / or personal injuries, then she's got bigger problems ahead.
If the boyfriend had his own policy of insurance, then a claim could be made against that policy for her damages. If the other party was damaged, then they can make a claim against that policy also. If there was no policy with the boyfriend, the other party can also make a claim under their "Uninsured Motorist Coverage" and, after their insurance company pays them, their insurance company will go after him and your sister for reimbursement.
IAAL