We had a niece who went through this. She’d come and stay with us for a month every summer while she was a kid, and there were a couple of summers where she just…smelled like poop. The entire time. It was pretty unpleasant, not only because of the smell, but because we could tell she was really embarrassed about it. She’d been very outgoing and social in years past, and was suddenly hiding in her room all day. Wife helped her wash her clothes as often as needed and tried to make it not a big deal.
We never heard much about how her parents were dealing with it, but we knew she was seeing a therapist. Something eventually worked, because we got a few more mess- and stink-free summers with her before she graduated from high school.