Typically insurance coverage follows the vehicle first and driver second. If your sister's insurance company finds it acceptable for her friend to be driving the vehicle at the time of the accident that coverage will be primary. If there is more coverage needed the driver's insurance is usually found to be secondary or excess.

If your sister does not want to put the claim through her insurance company but instead go through the friend's insurance and the friend agrees you may be able to claim it that way. The best way to find out for certain is to have the friend contact her insurance company and explain the situation. In the end it might have to go through your sister's insurance first but by contacting the insurance companies you will find out for sure.