Every thing I've read on them mentions nothing about them being poisoners. Some countries even eat them. But they can be detrimental to the equipment. It's called bio-fouling. Which it the undesirable accumulation of algae, plants, animals on wet surfaces. Otherwise they will attach themselves to things like our pumps and possibly destroying them.
They won't hurt your friends bait tank. They most likely won't live long. But do tell him to be careful as the Barnacles shell is very sharp.

That's all I know