I just have traditional Medicare; the Advantage plans in my area were too restrictive and expensive. But I read a LOT about them before I made my decision. They are HMOs, basically, and you have to be very careful about going through proper channels, getting preapprovals for everything and, as you experienced, going only to doctors/hospitals that accept the plan. (My doctors didn't accept the Advantage plans in my area, which is another reason I didn't go with one.)
You should be able to check online at the Medicare.gov website to find a local doctor that will accept that specific insurance -- have you tried that? Also, be very careful about his prescriptions -- it's easy to be prescribed something that's not on the formulary and end up paying full price (and/or ending up in the donut hole).
As for billing, he should make sure to specify the Advantage plan he's using when he checks in for a doctor's appointment. Medicare billing can be tricky; I've had to go back and talk to the billing department a few times myself when the wrong code was used for my cholesterol tests and Medicare was rejecting it.