I borrowed a friend's 3DS to play Pokemon Y until I got a used one in the mail, so I think I can speak on this question. Even though X and Y require an SD card to play, all of your multiplayer info (Friend Code, your friends, etc.) is stored on the 3DS's memory itself. The 3DS does come with a function to transfer its memory from one 3DS to a separate one, but it will delete the data on the original 3DS. If you are the one who has been more active online, I would think it would be easier for her to use the new one, however, its possible for you to keep your current data on either 3DS (but not both!).
As far as using Blaziken (who is also one of my favorites BTW), my understanding is that people are currently keeping the same tiers as gen 5, with just the obvious new legendaries uber (Xerneas, Yveltral, and I'm not sure about Zygarde). However, that only pertains to battles with fellow Smogonites and friends who agree to ahere to it. When playing on random wifi battles, you play by Nintendos rules. This means no sleep clause, no evasion clause, and a slightly different set of ubers (Latias, Latios, any event-exclusive pokemon like Jirachi and Celebi are banned, whereas Garchomp, Excadrill, Thundurus, and Blaziken are all legal).