That poor lady. No mum ever anticipates that. My heart broke for her when you talked about her parents and how they feel and her DH....she might have short term memory loss, but I bet she still understands. Perhaps your DH might remind everyone she didn't choose this for her life.
In my experience with brain cancer it is sadly very quick usually, though there are a myriad of different brain tumors and each one is different. It will also depend on how treatable the tumour is in terms of radiation/chemo. Brain tumors are very hard to treat though.

I hope your journey is peaceful mama.