They should have insurance, i'd demand a new lawn.
You don't need to water 3x a day, even fescue in the south. You do need to baby it in summer months as I'm sure you know.
I suspect they put out product meant for warm-season grass on the fescue, something that lingers like metsulfuron, would damage or kill a lot of the cool season grasses. But if you don't know, you couldn't prove it.
One spray can hurt a tree, but one shouldnt have killed it unless they did something silly like imazapyr.
I was watering 2x but found when I added a light late afternoon cycle that it improved. Oklahoma's weather is hard. One day it's 40 degrees the next it could be 100 degrees so I have to adjust my watering quite often. But, I found what works for it.
This company has sprayed for years and sprayed my lawn for at least 4 years. They do all the bermuda lawns first then come back to hit the fescue lawns. They use a different container for their herbicides and it is too difficult for them to switch back and forth.
I may never know what happened.