The body wants to dump heat, and evaporative cooling works whether the body squirts the water out itself (sweat) onto the skin's surface or whether you spray it on with a spray bottle.

After my grandma had chemo and radiation for cancer she had a very hard time with heat because she couldn't sweat right. I remember she had a little battery powered fan and a little spray bottle. Seemed to work for her.

If your mom is uncomfortable, maybe have her try wearing a wet shirt while riding? Dribble water in through the vents of her helmet?