I don't see anything that would run afoul with anything.
At worst looks tacky? At best generates growth, and assuming professionalism is maintained, the family/friend getting a consult will be performed with usual lens of medical necessity. So should be fine, IMO.
My first thought is that it could be considered bartering, albeit a service rather than a good, and thus should be reported as income in the amount of the cost of the "free" appointment.
Seems like a boundary issue. Giving free services to incentivize referrals. What if someone gets you 10 new patients? What will your subconscious now feel it owes them? They’re making you a lot of money off repeat business, much more than the one free appointment you comp then.