you mean this?? http://www.siliconera.com/2013/03/06/send-virtual-armor-to-your-virtual-friends-in-sword-art-online-infinity-moment/
you need to max the stats from the Attack, Defense, or Buff in the Heroine's status screen.. just talk to her and pick the second option (I think, sorry I forgot) when she is in your party to see her stats..
how to max the stats, I'm gonna give some very general idea on how to do it, just praise your partner during battle.. press R to bring out the command AI menu during battle, after that see the top list of the command menu that is the command for praising your partner, press R again to praise her..
okay, during battle you'll see that after your partner do some sort of move (it can be sword skill, battle skill, skill chain, etc) the command list for praise in the command AI menu change it's name and highlighted in different colors.. red, blue, and green
if you praise her when it's red that means you'll give some stats point to one of the Attack stats.. the same thing goes for the other colors.. Blue -> defense stats, green -> buff stats
after one (yeah, only one) of the stats is maxed you'll get her particular battle type costume..
I think there are also some sort of other requirements like the friendship rank or you need to be on certain floor (in other words after a particular scene) but I don't know for sure.., since my friendship rank is already quite high (rank 2 and rank 1 mostly) and I'm already on the 84~ish floor when I tested it
apparently you can also unlock the battle type costume on the new game+ but I haven't tested it myself yet.. busy with something else and busy with other games too..
oh right, how to change the costume.. just ask her to join your party and then talk to your partner and pick whatever that choice is for seeing her stats screen (1st choice or 2nd??) and when you have access to her costume instead of going to her stats screen she will give you more options to choose.. one of the choice is for the stats screen the other is to change her costume..
note that you can ONLY change costume in town.. pick the costume and then MOVE to the other area.. example you ask her to change costume in the teleportation gate area, you need to move to shopping are for the changing to take place..
reason? apparently you need to change screen (or whatever) and auto save in order to apply the costume.. --- "Well, It's the Black "Waltz," right? You think there'd be three, right?", ~Zidane Tribal, FF IX~