It will all end up depending on whether they think it's a risk to let her in. When I underwent the IR1 process I entered the US multiple times on the ESTA (although only during the petition stage, not when it was at the NVC). I didn't have a job at the time but did have a lease in Belgium, which where I come from. I never hid that I was visiting my wife and when asked I was open as well about the fact that I was actively in the process of getting a spousal visa. I never had any problems getting in but obviously also had a history of compliance (as in, leaving perfectly on time, every time) and a return ticket which likely helped me.
So, it's definitely possible, even with a lower amount of ties to your home country, but it will all depend on the CBP officer to believe she will leave on time and for her to convince them that she indeed will. Having recently quit a job is likely to raise red flags though.