Re: girls and heels
Start her on a Hobie Cat. Worked for me, I don't get edgy til at least 30 degrees of heel. Of course, we go faster with the flying hull just skimming the water.
Joking aside, there's something to be said for having what she's worried about happen or proving that it won't happen. On a dinghy, people are much more comfortable after a few capsizes and recoveries. On a keelboat, you may find that burying the rail and surviving (without any issues or panic) helps sailors be comfortable with 5-10 degree less heel than that. Knowing what it takes to capsize a large dinghy and how it feels before it goes over will also make you aware that at 20 degrees you're nowhere near capsize.
And honestly, if you're not going faster or having more control, why heel more? The right angle of heel depends on your boat but if it's just to feel saltier, maybe she's right. If you're doing any extended cruising, you may want to look into the relationship between degree of heel and crew exhaustion. Turns out that just being heeled for hours can wear people down. The research was done mostly with Navy crew, so it's not just that they weren't tough enough or used to it.