making a guard attacking a mob that is fighting a player is different than making a guard attacking a mob on sight.
I ve read in Sandy's post :
http://www.eqemulator.net/forums/viewtopic.php?t=14391 that agro is working that way : if a mob have in range another mob of the same faction that is fighting, it takes the second mob's target as target and attacks it.
So if the guard and the player have a common faction and are 'friend' enough in that faction, the guard should support the player against the mob.
This remains obviously a pure speculation, but i tho remember, (perhaps from Winters Roar), some guards saying : "Die, scrum !" and helping low lvl chars in toxxulia