I started going bald in my early twenties. Took alot of abuse and ribbing from the guys and from those who couldn't believe my age and the hair loss I already had. By 30, I was shaving my head. I was called Yule Brenner for a while or Curley and Kojack. I chuckled and moved on. Deep inside, it hurt some, but who's gonna stop me right? The before you knew it, shaving one's head became cool. Movie stars, pro wrestlers, MMA guys, you name it, guys shaved it off.
My point is, let her shave it if it makes her feel better about herself. If it brings her more confidence than the thinning hair, then be confident. Its her move, her head, her emotions and you, as Dad, are along for the ride. How you help her with this ride is entirely up to you. Your support and yes, maybe your shaved head in an empathetic attempt to ease her feelings is all good. At some point, she may want a wig, as my Aunt did when her chemo took hold. People with cancer have a roller coaster of feelings that bounce all over the place. Its hard to be them, its hard to follow along because you never seem to have the right answer. Just be Dad, love her in every way and do what ever you have to do to make her day brighter.
You and your family are in our prayers. Today, my shaved head takes on a new meaning. Tell you daughter a guy in Jersey who's been shavin' his head for 22 years is on her side. God bless all of you.