There's no way to force the parent to wear adult diapers but speak with them and let them know that they will feel more comfortable wearing the diaper and not having wet pants or wet clothes.
Or if they have fecal incontinence then they will feel better having the poop in the diaper instead of getting the poop all over their pants, clothes, legs etc.
You can't force the parent to wear adult diapers but you can try to convince them.