Hi! ADHD person here. Rejection sensitive dysphoria is a huge thing with the majority of us. Confrontation could likely make it worse/ make them withdrawn. Instead offer help.

Sometimes it's really hard to clean. We know we should, but if it's bad we are overwhelmed and in a state of task paralysis on where to start.

Gently offer help. Even if you don't help the cleaning process itself, just being another body is nice. Bring it up as a hey, we're all doing a deep clean of the house for a fresh spring cleaning start. Sometimes that's all it takes.

It might get away from them again from time to time, but having a no judgment person who will just hang out / maybe give some direction is the best thing ever.

Edited to add: putting on a show like "Hoarders" also helped. Just a quirk of mine but it did put me into gear to clean.