As messy as legal malpractice lawsuits can be, the base relationship is usually straightforward: a lawyer facing accusations of wrongdoing by a client. But a case before the District of Columbia Court of Appeals this month is testing the limits of who can be considered a client and when nonclients can sue for malpractice.

Boston-Maine Airways Corp. is appealing the dismissal of malpractice claims against Sheppard, Mullin, Richter & Hampton. Boston-Maine claimed that Sheppard, which had represented several of the airline’s sibling companies, was to blame for a poor outcome in proceedings before the U.S. Department of Transportation. The company accused Sheppard of foul play after the firm failed to immediately disclose that it was advising a general counsel, shared among the companies, who was facing allegations of misconduct.