Can I buy Daylio for somebody else?
Daylio is distributed as a an app for private use via Google and Apple stores. This means we play by their rules.
Apple AppStore provides option to buy a gift. For more read official tutorial: Send App Store & iTunes Gifts via email.
Android Google Play Store does not allow simple app gifts. The easiest way would be to buy a Play Store credit. For more read official tutorial: Where to buy Google Play gift cards.