no. the 12000-3600 (difference b/w billed and allowed) is nobody's responsibility.
she only need to pay her share (co-insurance, office visit cocay) of the $3,600 fee schedule amount (insurance company allowed amount).
If her insurance is HMO, maybe there is no co-insurance, just office visit copay, $10 or $20. She needs to read te benefit.
Let the doctor send the bill to colection agent, then she has evidence to sue him.