I worked for a lawyer and have seen some of these situations. There are no winners. The family is in Hell. Absolute Hell. Many times, there are no symptoms of mental illness until the child is an adult 18 - 25. They have raised "normal, happy" children and do not see this coming. The future they have predicted for their family is blown up and probably won't get better. For many people, these episodes are a way of life. ( There but for the Grace of God, go I.)
Absolutely, speak with your lawyer and follow that advice. I am not giving you legal advice.
The police and emergency services will be involved. You want them to be involved because that could be the only route where she gets help.
As she is over the age of 18, nobody can make her go to a Dr, get a diagnosis, take meds or go to therapy. When she is having an episode, if she is a danger to herself or others, the police can take her to the hospital for a 3 day lockdown. Hopefully, the patient can be stabilized a bit. It is possible, but not guaranteed, that the hospital will be able to keep patient for a week. To do this, the hospital is hiring lawyers and going to the courts for emergency orders. Hopefully, while in hospital, someone will be able to convince patient to go into a treatment program. If the patient chooses to not seek care, there is nothing anyone can do. Generally, this whole cycle will repeat. This can go on for years. ( This is a negative aspect of having civil rights.)
I'm not giving legal advice. If it were me, I would consider reaching out to local police via non emergency phone. Simply, tell them that you are the owner of the property at this address and there have been some calls for emergency service. Tell them that you know this is a private family matter for them to handle. Ask if, as the owner of the property, there is anything you can do to help them? They will probably say No. But, you will have established a connection that may be useful later. They should have your contact info, just in case. This is a 3 min phone call. They will not give you much info about what is happening so don't ask.
If it were me, I would pay them to leave. Tell the parents that you are getting complaints, etc. Would they be better off with out shared walls? If they would move, I would give them every dime of their deposit, kick in another $1,000 and pay for the moving van.