it will evolve it's friendship with you is about max at night. Try to give it a shell bell to help or don't let it faint in battle that will just reduce the friendship.
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it depnds were u evolve it
You buy Happiness in nighttime at the shop.
You need to make Eevee love you, and then you evolve it at night.
Trade it to ruby,sapphire or emerald have them evolve your evee at night then give it back to leafgreen.
Level up an Eevee with full happiness at nighttime (after 6pm)