Her response was "I'd just rather go fast." (As in it's more fun to go fast without thinking about technique.)
Developmentally, a low intermediate who's doing a series of braking rotated sideslips down the hill is reinforcing a lot of bad habits until they slow their movements down and focus on building effective patterns (at least on the pitch we were skiing). But as DanBoisVert, CarlR, TomB and RR comment, the investment in developing at this level may take a backseat to the immediate fun of going fast, particularly for an occasional skier. And, like music gal hinted at, perhaps there was an element of wanting to "keep up" - even though it was totally unnecessary. It's an interesting contrast as I've skied with other friends who really aren't concerned with trying to keep up.
Has she booked another lesson? Now that you know what motivates her, it should be easy to tailor her next lesson(s) to her goals. I bet that if you free-skied with her and skied fast enough that she'd fly off into the trees if she tried to keep up using linked sideslips, she'd be more receptive to your instruction. Just keep emphasizing why the things you're helping her with are important when skiing at speed.
If that doesn't work, maybe find her an adult racing program with a coach?