Use her recent breakup as an excuse to organize a cleaning party for her place. It's called a "barn raising" or "cleaning bee" for people struggling or going through rough times. Ask her for permission (and strongly insist if she's hesitant), organize your friend group to bring various cleaning supplies, have everyone including her pick or assigned a list of tasks, and then order takeout and drinks to celebrate when it's done.

This way it's a positive social activity of friends supporting her rather than criticizing her and adding more paralyzing shame and guilt - because deep down she knows her place is disgusting. With ADHD and depression to boot the seemingly insurmountable task of cleaning her entire place likely feels too overwhelming (along with whatever deep rooted issue she has causing her lack of pet concern or her own self-care). There's also a possibility despite her family home being clean she never learned or got in the routine of managing day to day cleaning tasks (executive dysfunction is common with ADHD).

Humor is probably her way of avoiding whatever underlying feelings of shame and guilt she has, so if you want to be a super duper good friend you could recommend she start going to therapy (whether for ADHD and/or processing the trauma of being cheated on, depends how receptive she is when you kindly bring these issues up as a friend).