Short answer Yes.
Your friend may well be earning what he says but for you to come on here and ask us if that sounds right I get the impression you think he is fibbing. As you are working now you might as well canvass a few areas and if you get any work try to do them at the weekends, and if you get inundated with work then you should consider handing in your notice but as things are at the moment I wouldn't pack in a job until I had enough window cleaning work to cover my monthly outgoings at the very least. (mortgage, rent, bills etc etc) Only time will tell if window cleaning is right for you, could it be the best thing you ever done? It very well might be, but I would build up some sort of work before jumping in at the deep end and packing in your job, but believe me lots and lots of people will disagree with me and say go for it, only you can really decide whats best for you. Carl stick the question on the window cleaning section, you will get lots of replies. Best of luck to you though if you do go ahead with it