Generally, a malpractice suit requires not only objectively negligent conduct by the lawyer, but actual damage to the client. I think it could be argued that even if they have objectively incompetently handled this case, it never had any merit in the first place, so no harm, no foul. Of course then they have to argue they deliberately took a completely meritless case.
It generally takes the opinion of another lawyer to establish malpractice, although I'm not sure if this is the case for more obvious cases of malpractice, like simply outright failing to file a notice of appeal on time. A bar complaint might go further. Even pro bono counsel has to work to some objective level of competence.