I worked in a nursing home when i was a CNA and i seen this type of thing a lot most of the time it was a fungal infection and it needs to be cleaned and dryed everyday also the area needs to breath so not having skin to skin contact where it can sweat putting a dry cloth between her breast and her chest. Also the briefs it would be wise to air that to at night like the doc said. Also i would see if it is staff because i have seen that to like the other posters mentioned. I hope she feels better soon i know it can be very painful and very irritating. Also does it smell? This is also a sign it needs to be aired as well. she can get ulcers if the rash turnes into sores. You are a wonderful woman to help take care of these women!
It takes a very special person to do this and i commend you for it.! Please keep us updated and make sure that nurse and doctor really look at her and not just look over her this could get serious if not properly treated