The girl is an insured under her parents' policy since she is a family member as defined on the policy. She is also an insured under your insured's policy because she was using a covered auto. However, the exclusion that eliminates liability coverage for any insured using a vehicle without a reasonable belief that he or she is entitled to do so is applicable here. A fourteen year old knows or should reasonably believe that he or she is not old enough to get a driver's license and so has no reasonable belief that he or she is entitled to use a vehicle. If both your insured's policy and the friend's parents' policy have the reasonable belief exclusion, the girl does not have liability coverage under either policy.

As for the question of whether the liability coverage under the friend's parents' auto policy comes in as excess over the med pay coverage on your insured's auto policy, the answer is no. The other insurance clause of the med pay coverage part is relevant to other applicable auto medical payments insurance, and not to any possible liability coverage. If, somehow, the friend's parents are held legally responsible for the injuries suffered by your insured's son, the medical payments made by the son's insurer can be subrogated against the friend's parents' liability carrier. The reasonable belief exclusion applies only to the girl and not the parents, so if they are held legally responsible, they would have liability coverage as named insureds under their auto policy.