I have always assumed that charmed pet xp stealing works the same as mage
pet xp stealing. In which case, the pet takes 50% of the xp if he
outdamages the entire group. That is highly unlikely. However if that was
a concern you'd merely let charm drop before the other mob dies...

If they really wanted to improve charm, I wish they would put our charm cap
up to 55 or something. Its annoying to lose one of our best abilities as we
reach the pinnacle of our career. I am far from the forst person to say
that though... Or at least give me back the old resist message (your target
is too high for this song or something) so I could learn which mobs were
charmable in high level dungeons...