If you're not grouped with the warlock, make sure the lowbie tags the mob before the pet hits it, or they get the kill and the lowbie gets nothing.

If they are grouped, then first hit means nothing, as long as the mob is damaged by a player and does not die entirely to the pet (which results in no exp's and the target is not lootable).

If the warlock is 10 or more levels ahead of the player, then the player (provided they tag the mob first) will only get the percentage of experience equal to the percentage of health damage they deal. For example, a mob has 100 health, the level 7 lowbie does 8 damage, the level 80 pet comes in and hits for 344, which finishes the mob off. Since the warlock exceeds the level 7 by more then 10 levels, the level 7 will only get 8% of the experience of soloing the mob.

It's still a very fast way for quests which require you to kill "x" of a mob, or to loot stuff off of a specific mob type. Or for mobs who are in the way of your quest item etc.