Please extend my sympathies to your friend! Unfortunately, that is an entirely legal and plausible scenario. Insurers are legally able to cancel or refuse to renew policies whenever they see fit.
Reasons an insurance company will drop a client typically differ from state to state, policy to policy, and insurer to insurer, but the most common reasons include missing too many payments, having bad credit, or—like in your friend’s case—filing excessive claims.
There also are numerous reasons out of a client’s control—like, for example, if your insurance provider simply changes their business strategy or if a previously covered neighborhood becomes riskier to insure.