Constant and involuntary rubbing of feet

This might not belong here, but I don’t know what else it may be linked to. Someone I’m close with has this “bad habit.” Or at least that’s what she calls it, but as I was just thinking about it, it sounds like much more.

She calls it “cricketing”, and it’s where she involuntarily rubs her feet together while she’s trying to sleep. If her feet are in any way touching, she rubs them nonstop. Socks don’t stop it, so she has to have a pillow, or a thick layer of blanket between her feet. But, that’s just so she can fall asleep. She also tosses and turns in her sleep, and the pillow eventually falls away from her legs. And as soon as her feet touch, even in deep sleep, she rubs them together. She’s got faint scars on the tops of her feet even, because there have been a few times that she’s rubbed her feet together so hard that she rubbed off her own skin.

She says this is something she’s done for as long as she can remember, even in very early childhood. It’s something her father always did, and still does do. Is this mental health related? And if it isn’t, does anyone know what it could be caused by? What can she do to search for relief from “cricketing”?