Since your friend is having early memory loss, ask her what she prefers. Do not correct her publicly in front of other people because it can cause her embarrassment. One of the emotions people with early dementia or Alzheimer's experience is shame and embarrassment, even though they haven't done anything to get the illness and can't do much of anything to control it. We never want to feed into that emotion by correcting them in public, where it disrespects their emotions and ability.
Ask your friend if she would like reminders when she forgets something. If she does, use common sense and only offer reminder's when it really matters or you will feel like a nagging parent and she will feel like a child who can't get anything right.
You can always ask your friend if she would like you to help her set up reminders around her home, such as a calendar with her scheduled appointments and activities, reminders for hygiene, medications, date and time, phone numbers of close family members, friends and emergency contacts. If you can put photos of each contact with their name and number that would be very helpful.
Your friend will feel like you respect her and understand the difficulties she's having if you just talk to her about her wishes. Just because people have early memory deficits doesn't mean they still can't make decisions. Let her know you want to help her as much as possible prepare for further memory loss, then leave it up to her what she wants.
This kind of conversation will strengthen your friendship and provide her with the dignity and respect she deserves.