On a very basic level, much of a sylveon’s requirements are very similar to an eevee’s—and, by extension, many of eevee’s evolutions. Sylveon eats the same sorts of food (either kibble or a meat-heavy diet), requires both walks and regular battling, and will gravitate towards the same sorts of toys (including chew toys, squeaky toys, and soft toys). In fact, in many ways, sylveon is among the easiest eeveelutions to care for, as unlike eeveelutions such as leafeon, vaporeon, glaceon, or flareon, it requires no special care according to its type. Fairy-types are generally agreeable and adaptable, and although they may come with mischievous streaks, they’re generally easier to domesticate than even normal-types. This, along with their typically cute appearances, is why they make popular pets in the first place. All of this, of course, is simply to say that sylveon in general are easier to care for than one might think. It just requires some basic knowledge of how to care for canine pokémon.
However, in your case, there may be a few complications that are far more important to address. You see, sylveon evolve in response to a heightened sense of affection. To put it another way, affection is very much like happiness, so sylveon has a lot in common with espeon and umbreon. However, espeon and umbreon simply evolve due to happiness. In tamed situations, this means “whenever the eevee in question feels an overwhelmingly strong bond with their trainer.” Tamed pokémon, after all, derive their happiness from spending time with their human partners. However, happiness is actually a lot vaguer than sylveon’s evolutionary requirement because happiness doesn’t necessarily mean closeness, either. Even in tamed settings, there are rare cases in which an eevee may evolve via happiness derived from time spent with a mate, from pure enjoyment of an activity, or even from a really good joke. It’s just that it’s easiest to achieve the exact level of euphoria eevee needs to evolve by allowing it to feel love for its trainer.
Affection, meanwhile, is also a requirement derived from the bond between a trainer and their pokémon, but it’s more physical and specific in nature. Sylveon are extremely rare in the wild because affection is pleasure derived from being taken care of and loved. An eevee that achieves sylveon’s evolution requirements is typically cleaned, fed, given a home, and showered with love (that is to say, given physical forms of affection, such as petting) on a daily basis. Affection isn’t simply happiness, in other words, but rather an overwhelming sense of belonging. This feeling may occur if an eevee feels strongly bonded to their mates or packs, but the feeling of loyalty an eevee experiences as a result of human affection tends to be a lot stronger than anything it experiences in the wild.
I’m sure by now, you’ve can gather why I’m telling you this, but for the sake of clarity, allow me to put it this way. You’re being given a pokémon that evolved due to his strong feelings of belonging, instilled in him by another human. Trading itself can be a bit tricky, as it requires you to build trust in a pokémon that doesn’t know you, but this is going to be especially challenging with your new sylveon, as you are, at the risk of putting it far too bluntly, not the trainer who helped him to evolve.
That having been said, it’s going to be especially important for you to spend as much time as possible with your new sylveon. You need to work especially hard to build his trust in you, and you need to emphasize the fact that although you cannot replace his original trainer perfectly, you will still love your sylveon as much as she did. Pay attention to the routines your friend had maintained with her sylveon, and try to follow those same routines when he comes into your care. Make your new sylveon an irreplaceable part of your life. Ensure he becomes well-adjusted to the rest of your team, and set aside time each and every day to give him affection.
In other words, go into this new relationship realizing that sylveon exists due to the affection he received from its trainer. Don’t let this discourage you, either. Let it remind you that the trade may affect your new sylveon in negative ways and that it’s important for you to reassure him as much as possible that he’s still loved.
Once you get past the first few weeks and once your sylveon becomes adjusted to you and begins to trust you, you’ll find it will get easier to care for him. (As I’ve said, sylveon’s list of daily requirements is far shorter than those of most other eeveelutions.) Just try not to diminish the amount of affection you show your sylveon (as in, continue making him be a part of your life), and you should be fine.
Good luck, anonymous.