Home remedies: A good one might be turmeric. Mix Turmeric with water into a paste....and place on the affected areas. This does stain so wearing white underwear will be an issue....but black old raggy ones might work! lol Turmeric is anti-inflammatory, and it pulls out the bacteria etc. You will have an orange bum when using it...but if nobody is going to see it then bottoms up and enjoy! :)
Why do you want a home remedy? Why not use proven ingredients? I would usually put the client on Lelexo's mandelic scrub and their Revitalize cream which is a body-strength vitamin A. I might also do a few peels and/or add a PFB roll-on to the mix. It's unusual to see acne on the behind without any breakouts anywhere else (face, chest, back), so I'd be treating them for that as well. All the usual lifestyle recommendations would apply - no fabric softeners, frequent towel and sheet changes, ice the affected area, eat low on the glycolic index, be sure they are getting enough dietary or supplemental omegas, vitamin A, and zinc, and don't consume excessive amounts of iodine (iodized salt, some seafood and seaweed, excessive dairy).