It's not a bobbitt although it's in the same genus Eunice. Most of them are harmless, even some of the big ones. What an individual eats depends on the species and the individual. Some species are strictly herbivores, some eat detritus, some are scavengers, some are predators. If they're not keep well fed some will go after items that aren't part of the normal diet - just like most inverts. If your friend didn't have any mysterious deaths in his tank before giving you the rock then the worm should be safe.
Go ahead & feed your anemones. Both male & female eunicids are required for a successful spawning so you don't have to worry about reproduction. If you cut the worm's anterior part off close to the head it will regrow a new body and the body will regrow a new head. If the cut isn't within a certain distance of the head then the rest of the body will not regenerate, it will just die.