That information would be in her obituary or on her death certificate. There are a couple of obituary volunteers at freepages.genealogy.rootsweb.ancestry.com/~obitl/volwa.html Scroll down to Cowlitz Co.
She may or may not have had a death certificate. They weren't required at that time by the State. You can check with the Cowlitz County Recorder, co.cowlitz.wa.us/auditor/recording.htm for a copy of her death certificate but they might charge to look it up.
There's a list of indexed cemeteries at the following web site: