This is not the “boss” but the manager, so there’s probably someone above him. Your friend should talk to whoever is above the manager. And they also should talk to the co-workers lawyer who will be very interested. That action changes things from harassment which may or may not be proven to “perverting the course of justice”.
Reading comments, I’ll repeat that this is one manager, not the company. And telling someone about an attempt at perverting the course of justice (which in the U.K. carries a lengthy jail sentence) is your civic duty, not something any company can hold against you.