I went through much the same, many years ago, coming home to find 2 hens dead, 2 so injured and traumatised we had to have them pts, and just one alive - she'd been laying when the attack occurred, so the dog hadn't spotted her.
We'd thought fox at first, but a neighbour had seen a Jack Russell loose... The owner apologised, saying his dog had been bored as the weather had meant they hadn't been out working. Well you should have taken it for a walk, then! We thought, but didn't say. The owner offered to pay for the hens but didn't, in our view, seem to think it could never happen again, so we made Chicken Knox, 6' fencing, dug into the ground and with three strands of electric, and our previously completely fre range chickens got barred up when we went out. It kept them safe. You do need to check the charge every day, of course. Use a blade of grass to test the wire - it's a poor conductor, so you get a much reduced belt through it.