As a sub contractor YOU work the strings not her, she has really found out how much shes getting for free and is afraid of loosing you. So she is trying to domainate you I wouldn't be surprized that you find when you look for new clients she has already started submarining you.
My hubby had a contractor (hardwood flooring) who tried this stuff to so we just had the office/home phone on a machine , got a cell that only family had the # for. This guy would call as hubby was trying to get the floors done, well not getting done, He would talk for an hour at a time . If I answered the phone he'd want Tim and then want to know where he was and if I told him what job (before I learned about him) he'd drive out and pester him there. Finally getting the Machine and no call backs until 11pm waking his 6 month old (rotten aren't we) he stopped using us no lose as in a year we got only 2 small floors and then all the people did was bitch about it. People with no money trying to have a expensive floor they can't afford,then trying to use the cheapest crapest material. We recently hear that he started his own flooring and what we have heard is not good, in fact we have be asked to redo his screw ups. We have pasted them on to another company who can afford to do them Bigger company.
You just have to put your foot down, if you lose her as aclient and freind you just replace her . I don't like working for freinds, I'm polite and freindly to our clients but we do not socailize with them. If we do work for a freind a contract is made up explaining all possiblity and the out comes, also if changes are made in the process they are noted and intialed by the client just so there is no problem. (on these job I usually spend more time on the site than others)
Hope some of these ideas help. You aren't alone in this kind of thing. Just part of owning your own buisness.