Unfortunately some groomers get really territorial. If you were busy enough that she is booked out for weeks I can understand a new hire. That shouldn't have resulted in any issues. However, when you bring in someone and your first groomer isn't full yet....It causes serious anger problems with some.
BUT you only hired someone to work the "off" hours and I do think that your groomer is over-reacting. If she doesn't want to work those hours its up to you the business owner to find ways to either let things sit empty or make more money. I have worked for shops that hired groomer after groomer. All the older groomers would go from 8 dogs a day to having 4-5 each and hoping that someone called the same day. The owners would hire for the same hours and I think that was a bad idea! Especially since we only filled up day by day. We didn't even have enough tables(7) at times,3 dryers and only two tubs. (With 12 people!!)