Try a synthetic, if it works for you sell your badger on B&B and avoid all the acrimony with the GF.
I make my own brushes.
The fact is, the badger that produced your brush has died. Your gf stopping you from using it means that badger died in vain. Better you use it than discard it. I still use the leather shoes that I bought before I went veg.
I make my own brushes.
Why not just use the badger? If she loves you, I am sure she will be able to respect you.
One post above said some PETA folks think having a pet is animal slavery. I'm almost positive that my cat views it the other way around.
-Andy