I do a little bit of work for friends but not much. I do them under totally different conditions from my business though. Either I do them FOC as and when I feel like it OR, if they are paying, I just tell them to let me know when they want them done and I will fit them in when I can. We are talking about once or twice a year at most. That way, silly things like a few bits of glass don't get in the way of friendship. I have quite a few customers. I don't have many friends. I don't use the word friends loosely either. I'm talking about the sort of people who have let me use their sofa when I was to ill to look after myself properly/collected me from hospital after minor surgery or given me the keys for their house when they were away. The occasional free window clean for people like that really is no hardship and furthermore, it's not even expected of me.
Years ago someone said to me "If you do something for someone else, expect nothing in return. That way you don't get hurt." I've also seen in AA stuff about doing things for others but not getting found out or it doesn't count.