Dan,
I have lived the same situation with my wife. Where exactly the tumor was located and to where it is spreading right now?
What is your mother daily dose of Decadron?
Physicians are not wrong in think about drug induced psychosis. However tumor location could set this kind of reaction as well.
Cindy D makes an extreme important point when she cited quality of life and the way your physician treat these reactions. Despite being expected it should not be left unattended. There are plenty of drugs that could be used to ease this extreme anxiety, and provide your mother some quality rest. Discuss with her doctor the possibility to use some of the options presented. I talked about this issue with Diane (from brainhospice.com) and she told me that the majority of cases are treated successfully with Haldol.
In my case specifically it took a huge effort to catch up a consistent set of drugs that actually helped. Some doses that should be efficient for any one of us, did not scratched the surface when it comes to brain injury. It is important to be very well assisted by your neurologist in this process, so you can safely change or increase doses. Most of these drugs had what countless interactions and only a good physician can handle this correctly.
Please fell comfortable to ask anything you want. During these days I felt terrible, because no one else seems to have the same problem. I usually heard from people being more and more detached from reality without the outbursts of fury, and bizarre manias I became used to watch at home. After a while a lot of similar stories started to pop up.