You might consider waiting til you beat the elite four and have access to the fourth island. There you can breed eevees with ditto so you have enough to evolve all 3.
When is the best time to evolve eevee. I'm going to evolve it into vaporeon.
To do that, he'd have to trade it to a G/S/C game and have them train it at the right time of day.
Thats impossible to trade between a GBC game and a GBA game. If you want an Espeon or Umbreon... just migrate to D/P, it still has time-based events (ex: rotom...)