I'm not certain how these changes in our friend could have possibly happened overnight?

My husband and I saw one of our friends at Walmart at few weeks ago. He stated that he had just got back from seeing his doctor and was told he had "dementia". At this point, he appeared to worry about something and told my husband and I that he felt like he was going crazy. From then on, he was no longer able to live alone or care for himself. He has recently been hospitalized twice. The first time in the hospital he was told he had "dementia and kidney infection", This past week he got sick and was once again hospitalized, where he was only told "dementia". We are blown away with this. Can it really happen overnight? Confused here,