Do you have any doggy day cares? Those places are great. If they work long hours, maybe you could see if they could bring the dog over in the mornings? Explain to them about how sad your dog is, and that it would be good for both dogs to spend time together?

It's not entirely the same, but when I have to bring my son in from playing with his friends next door (he's three) I often give him a special snack or something, or let him play his Winnie the Pooh game or something. Maybe you could start doing something like that when the other dog has to go away, just to make the transition a bit sweeter.

I would set up a time for her to always meet/play with the other dog. Dogs like routeine, and if you always have them play together every three days or so, she will come to expect that and (I think) be less depressed.