Instead of only ignoring her try to combine it with this: look her in the eyes for 2 seconds, then very slowly close your eyes and look away. This is universal cat language for friendly intentions.

If you don't mind spending money on her get a Feliway spray and spray the places she hangs out. It's a hormone that smells like their mums did and makes them feel more relaxed.

She also needs a safe hideout. If the owner didn't give you a bed for her try casually leaving a cardboard box on its side somewhere.