If there is not a bounty for a BH to chase down then the only thing a BH can do is scan cargo or wait at the office for a escort job to come across the wire.
If he happends to pass by a pirate ripping a trader he should just make sure to avoid the wreckage and ignore the screaming. I will keep this in mind.
So no hiring a BH to protect you while your being attacked, that is for mercs.
But you can hire a BH to escort you just dont wait until after the pirate attacks you.
Dont expect a passing BH to aid you in any way unless you are a bounty hunter and at that I guess BH's should not expect anyone else coming to their aid.
So if there is a trader being attacked by a pirate then the various factions should act as follows.
Police - can freely aid the trader as long as it is within the jurisdiction of his house.
Navy - will contact police and inform them of the situation.
Merc - only if the trader is willing to pay whatever amount the merc states.
BH - can not assist at all unless there is an outstanding bounty on the pirate.
Trader - chances are will take the opportunity to make some distance.
Freelancer - tough one here. Could be hired i guess.