For Budew to evolve into Roselia, it needs to get from 50 to 160 Friendship in BDSP, which requires an increase in 110 for its Friendship. For those of you who intend on evolving Budew at a later point of the game, I'd simply recommend catching one in a Luxury Ball, giving it the Soothe Bell, and using an abundance of friendship-raising Berries and Vitamins. Before Oreburgh Gym however, these luxuries don't exist.
Prior to Roark, there is a whopping one Battle Item, which increases Friendship by +1. This Battle Item in question is an X Defense located on Oreburgh Mine which, according to Bulbapedia, is located at,
B1F, to the right of the main entrance
Besides that, the only methods for raising Friendship that you can access are leveling Budew up in battle (+3 0-99 or +2 100-199) or a 50% chance when walking 128 steps (+1). However, Pokémon currently at their met location have any increase to their friendship boosted by 1. Therefore, the fastest way to evolve Budew prior to the first Gym is Leveling Budew up at its met location, using the X Defense sometime during a battle, then going on to run around once gaining EXP becomes too tedious.
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