An ankle holster is definitely not a great place to carry your primary weapon. I've worn an ankle holster. Besides the slow access and draw, in order to be tight enough not to flop around while running or walking, it is also tight enough to reduce circulation a little bit. After six or eight hours, you are happy to get the durn thing off. And, since your leg is larger at the top of your calf than your ankle, the durn thing wants to shift itself down onto the top of your foot all the time.
The nifty little suspenders sold by some holster makers have a similar circulation problem. They wrap around the top of your calf, and have a single strap that goes down to the holster. In order to be tight enough to hold the gun-weighted holster up, it has to be a little bit tight. But, here Mother Nature works against you in another way, too. When the top of your foot is angled down in relation to the front of your leg, your calf is expanded. When the top of your foot is angled towards your shin, your calf is thinner. Thus, when you walk, the durn thing presses and loosens against your calf. Try sitting with your legs extended straight out and the top of your foot at 90-degrees to your shin in order to get some relief from the calf-wrapping strap. Oh, your legs are extended because you want some relief from the calf-wrapping strap digging into the back of your knee--its top edge.
Ankle holsters just aren't all that great an idea.
Maybe after she takes some classes and reads up on armed self-defense--real encounters--maybe she will change her mind and start wearing a belt. A holster for inside a purse might work for her, too.