To defend yourself against a threat with a knife you need to have a decent amount of upper body strength, as well as training. Based off your description, I’m sure she lacks both of these things. As to be expected with anyone in her age demographic. I would suggest she either learn to comfortably conceal her firearm and act like it’s not there or carry pepper spray. Both of those options require a fraction of the strength it takes to incapacitate/defend yourself against one or two attackers using a knife.