Is it wrong for kids and adults to wear diapers if they don`t need to wear them?

a female friend of mine has a 20 year old daughter who wears diapers 24/7.she normal and dosn`t need to wear them.she just like to wear diapers and use them for their intentent purpose.it looks so weired her laying on the bed at her age while having her diapers changed.i`m afraid if this continues to go on much longer the girl will become incontinence and have to wear diapers all the time if she isn`t already maybe regress into a baby again. i`v talked to my friend seveal time about this.she just saids her daughter enjoys wearing diapers as much as she enjoys changing her diapers.am i right and they wrong or am i wrong and they right.