What sort of insurance will cover me if my property is stolen out of a friend's car?

This really depends on place to place, as per covers offered.

There are addon covers to insure property in the car, which has to be opted for when taking the insurance for extra premium. However do check the exclusions as some do not cover certain items like jewellery, cash etc;

Also if its a laptop insured with your office or home insurance package, as portable item, it can be claimed.