Layering over the diaper will soften some of the sounds. She can wear other, cloth underwear on top of it, cloth diaper covers, bike shorts, and leggings as quick examples.
Aside, the diaper wearing community, for whatever reason, strongly prefers the loud, plasticy crinkle sound so most ABDL designed diapers cater to that. Instead of just buying the marketed ABDL ones you may look into “cloth backed” or “cloth like” adult diapers from medical providers. They are much quieter and generally more discreet.