Eevee evolves into Espeon during the day while having high friendship (When it levels up). So, in order to not learn Confusion, Eevee will have to evolve before level 15. Now, there are multiple ways to raise Friendship in Crystal, so I will outline them.
Leveling up where the Pokemon was met
This unique mechanic in Crystal is a very efficient way to to raise friendship until Level 14. It raises Friendship 10 points from 0-99 Happiness, 6 points from 100-199 Happiness and 4 points above that. This will raise friendship quickly enough until you reach level 14.
-You can get haircuts in Goldenrod City to raise friendship everyday. It varies based on the brother, but it is still good for a boost every day combined with other methods.
-Using Vitamins (Carbos, Protein, etc.) This raises your happiness by a good amount every time, but these are hard to come by.
-This essentially just leaves walking around. Every 512 steps yields you 1 Happiness point boost, so very inefficient.
Conclusion: Having the level bar at 15 gets rid of the most efficient method of leveling up where the Pokemon was met. A combination of the Haircuts and walking a whole lot should yield you enough Friendship in not an extraordinarily long time.
Source for Friendship Chart for Reference: https://bulbapedia.bulbagarden.net/wiki/Friendship