You can definitely give your Ava to a friend to use.
Before you lend or give your Ava to someone else, please make sure that you have synced the bracelet to your account in your app so that none of your data is still on the Ava. After syncing, all of the data (be it from last night or several nights) is uploaded and the Ava's memory will be cleared.
Once your give your Ava away, your friend would need to download the app and create her own account. All of the Initial Setup steps can be found in our FAQs under "Initial Setup".