If he a true professional who entertains at weddings on a consistent basis that is awesome. Total score. However, if he is not then I would be very cautious. Usually the outcome of these situations end with the friend heavily partaking in eating, drinking, and talking to people they have not seen in a while. It turns into more of a guest at the wedding who is playing music. They will not be dedicated, focused, or know the logistics like someone hired specifically for your wedding and who has their reputation on the line.