If they are just going out to kill a deer for some meat, I personally have no problem with it. If they are out killing many deer for there racks, then I have a major problem with it. I would guess in this situation they are killing for racks since they killed a nice buck. If someone is killing a deer just for some meat, they usually kill a doe. I would turn them in if tey are indeed poaching for racks!
I have a problem with someone killing anything thats out of season. No matter if its a doe or buck its called poaching.