Some people with Alzheimer's develop what's known as aphasia, which makes it difficult for them to pronounce words or make sentences. It's not that unusual that they simply stop talking
altogether. In those cases we need to be aware that they may still understand some or much of what they hear.
Human beings have many ways to relate to one another, of which the spoken language is only one. If you watch her body language you should be able to tell how she's feeling. Even if she cannot talk, you should keep talking to her anyway. Tell her stories that she enjoys, talk about things that interest her, read to her and sing to her. Don't be surprised if she's able to sing along with you. I've known people, who had stopped talking except in grunts, who still would sing every word of a familiar song. I've also had people suddenly utter a completely lucid sentence after months of silence. Keep her involved in your household. If she likes that sort of thing, she can still help set the table, fold towels, sort socks or clip coupons.
Remember, what matters most to your friend is to feel safe and loved.