I think the best point is he has the wrong attiude. He should be of the opinion that anyone who sees someone else in harm's way should act.

Althought it's not his responsibility to separate VFR flights, he can and should give conflict advisories on a workload-permitting basis. For criss-sakes he's staring at a radar.

If it happens on a controller's watch, you should at the very least say this is a lesson learned that conflict advisories are important to any controller who takes his or her job seriously.