Hi, Daisey. Here's one problem that applies to adults as well as kids: If we are doing well on our meds, we don't behave like people with bipolar. A new doc who sees us looking well may be very skeptical we even have the illness. But since we're adults we can communicate all those times we acted crazy or like we wished to die before we got treated.
It's very different with kids doing well on their meds. An 8-year-old is not in a position to explain to a doctor how bad things were for him at say age 6 before he was put on meds. The only word the doc has to go on is the parents, but a lot of docs unfortunately attribute bad child behavior to bad parenting. A lot of docs, despite clear scientific evidence to the contrary, still don't believe in the bipolar diagnosis in kids. So the doc says the kid doesn't have bipolar, and the parents may be too unsure to argue back.
Compounding issues is the parents may have come across some highly-uninformed media reports (and there's a lot of it) that bipolar is overdiagnosed in kids.
So what are you describing? The kid is going off his meds, and - no surprise - he starts acting out. He starts showing symptoms of his illness, which may be bipolar.
Now maybe the new doc is right. But your friend clearly needs a second opinion. Look at the events as you describe them: Kid goes off meds, condition gets worse, starts acting out. Clearly, if your presentation of the facts is correct, the kid needs to go back on meds. Maybe not the same meds or the same doses, but clearly some kind of meds treatment.
So here's what you can do for your friend: Strongly advise a second or even a third medical opinion - with a doc who is familiar with treating kids with bipolar.
Please be advised: I'm not a medical doctor nor any kind of child expert. Your friend's kid may or may not have bipolar. But clearly bad things started to happen when the kid was placed in the new doc's care, which throws a heavy onus on the new doc. So, again, a second or even a third medical opinion.
It is appropriate for you to bring this up. It is also appropriate for you to act as a sounding board for your friend. But, as you probably already know, you're on very shaking ground when you venture your own opinions on parenting to another parent. So, maintain the friendship, offer advice, but know your limits.
Hope this helps -