Not an REI employee.  You may be able to patch the window with some clear "tenacious tape" that will get you by    You can patch holes in the mesh with repair kits also.

But.... unless you sew the tent yourself I doubt it is worth it to fix it if the mesh is falling apart and even then...it sounds like the tent has been set up somewhere for a long time and has suffered a lot of UV damage so it is unlikely to perform very well if there is any weather. 

Tents do have limited life and while you might be able to get some more out of it for gentle car camping or as a kids play tent,  from what you say I would start looking for another tent.