If she insists on having a knife, then suggest to her a knife designed for her intended purpose... of getting the bad guy to back the hell up so she can employ the firearm. Something like a Ka-Bar TDI or the Benchmade SOCP dagger.
The SOCP has a very thin handle that will not alter a two hand pistol grip by much. The pummel end is a ring that you put your index finger in and makes for a very secure grip with great difficulty in disarming. This may not be legal for carry in some states as it's a dagger (both edges sharpened). They also make a set that comes with the actual dagger and a metal trainer knife that fits in the sheath and could possibly work as a kubotan type weapon of it's own.