Dog advice: My copperhead killing dog failed to attack one in my yard

My Aussie Shepherd, who is 4, was a copperhead killing machine. I trained her to kill them, but to ignore non-venomous snakes (she'll give them the once over, but won't kill them).

Ex-wife in residence almost steps on a copperhead. My dog ignores it, actually rolls on the grass nearby so ex goes back into the house.

Ex would know what a copperhead looks like but is crazy. Couldn't smell it (dog's way of discerning venom v. non-venom)? Home turf issue? Being at home with ex all day ruining the dog?

This is a dog that would go berserk over copperheads.