Ok... some of you might find this a bit "cruel" (really... it isn't!), but how about buying a water pistol, and shoot Lydia everytime she attacks Briggs (remember not to shoot her BEFORE she attacks... she then won't understand why she's getting squirted), and NEVER after the attack.

If they have to be left alone (like during the night, or during workhours), put them in seperate rooms (due to the fact that you'll have to squirt her EVERY time she attacks Briggs). Lydia will soon associated attacking Briggs with getting wet, and hopefully she'll stop soon...

What ever you choose to do, I wish you luck!!!