A squirrel found a robins nest in my yard today and destroyed two eggs. Will he come back for the remaining?

Leave it be. It IS likely the squirrel will be back, though. There IS something you can do, discourage the squirrel. You can TRY spreading plain ordinary dishwashing detergent on the tree limb leading to the nest that the squirrel will travel on. Anything greasy will work, like Crisco or lard. A friend of mine used vegetable oil and laundry soap to make a paste. The idea is to use something which will not hurt the tree but still will be something the squirrel will not likely walk through to get to the nest. Anything biodegradable would be good. The soap makes it taste bad. Ever seen a rat guard on the lines used to tie up a ship to a dock? Or the plastic collars used on dogs sometimes to keep them from chewing on an injury? You need something similar around the tree limb between the trunk and the nest. The collar needs to be about twice the height of the squirrel or it may be able to simply jump over. My neighbor did this on a tree in his yard with a nest. He got the collars on loan from a local vet, at no cost, provided he return them when no longer needed.