If a lawyer is intimating that some issues a witness might not like will be brought to light,without making a direct threat than it is probably legal,scummy but legal.
Need a little more info.
Specifically, I had one now former employee contact me in agreement that his employer was perpetrating a fraud upon the franchisees/business opportunity buyers. The lawyers threaten this employee with legal action and the employee then made public statements contrary to those he had put in writing to me.
I tend to agree with your assessment of "scummy", but not illegal. However, if the lawyer's actions help his client to continue to perpetrate the fraud on new victims, then might not the attorney be libel in a civil suit brought by the scams new victims?