Can we insist on generic drugs?

My mother is in a nursing home suffering from stroke and dementia. They have her on Seroquel and it is costing her out of pocket over $600 a month. We have asked the psych nurse practitioner to please consider a generic, lower cost anti-psychotic drug as my mother cannot afford the Seroquel along with other prescriptions, now out-of-pocket monthly at over a thousand dollars. To date, they have not changed it and keep saying Seroquel is the best drug for her. My own federal government health plan (Aetna) offers a list of several generic, lower cost anti-psychotic drugs for their preferred formulary so I know there are safe and legal options for her medication. We met with the staff in March and told them her drugs were costing too much and now, May 2009, she has reached her gap and having to pay all out of pocket. What else can we do?