Ants do not normally eat grass. Look for other causes. Too far north for fire ants(or so I thought until I looked them up online).
Its probably a different species than that found in the American south.
Contract this out to someone who can treat them. Boric acid ant baits are a possible method. If not, use an ant bait--wait--probably you cannot treat with pesticides in Canada--unless there is some sort of biorational product available--maybe an insect growth regulator. DE?
There is a good chance that the grass would recover given plenty of water and a top quality fertilizer program designed to increase grass density. Sod and or seed is another option.