It's ableist to associate sexual harassment with mental illness. And it's particularly disgusting to bring it up in the context of defending the harasser. Ableism causes disabled people to be sexually assaulted at higher rates. Ableism renders disabled victims near invisible.

If you only bring up mental illness and disability to minimize behavior like domestic and sexual violence, you're not being clever. You're contributing to a culture that throws marginalized people under the bus to protect the powerful.

https://disabilityjustice.org/sexual-abuse/ 

"having a disability was associated with an increased risk of sexual coercion (pressured sex without physical force) and noncontact unwanted sexual experiences (e.g., harassed in a public place, made to participate in or view sexually explicit material)." https://www.cdc.gov/violenceprevention/datasources/nisvs/svandipv.html

So technically disabled people are more likely to be subjected to that behavior. Minimizing it in the name of ableism is ridiculous and harmful.