Tarceva Prescribed for my Mum, after surgery and chemotherapy.

Hi was wondering if anyone would answer my question.

My mum was diagnosed with stage 2a lung cancer, and was given surgery and then 4 months of chemotherapy( cisplatin and this other one which i can't remember). after her last chemo she started to have headaches and blurry wisions, which was concerning so we took her to the hospital and then they did a lumbar puncture which found high pressure on her brain. after the fluids was taken out of her she felt better and after a while the symptoms comes back. she had lumbar puncture 3 times and then doctor prescribed her Tarceva, and somehow her symptoms these few days have seemed to become better.

My questions are, my mum's cancer did not spread and she also had various pet scan and ct scans, blood test you name it to confirm that, but the doctor only said they found abnomal cells from the fluids from lumbar puncture. prescribing tarceva means the cancer had spread right? could the cells be metastasis lung cancer(although the doctors are still not sure)? if so why high pressure on the brain? and if these cells were cancer cells tarceva would prevent them from forming tumours right?and since after the surgery all the test my mum took was clean and clear, nothing really wrong with anything except for the "abnormal cells", could tarceva kill off these abnormal cells once and for all and live normally again?