You only control the Friend while Pony Charm is attached to it. Control reverts back to its previous controller when Pony Charm leaves play (which can happen if the resource is removed via an event, the Friend is frightened, the Friend is discarded, or the Friend is returned to the player's hand: the Friend would go to the original owner's discard pile/hand/etc).
"The Friend you gain control of stays in the same place. If it's at a Problem, it's now on your 'side' of the Problem. If it's in an opponent's home, it will get moved to your home as part of PPP (this bit is in the rules update for set 3). ...cards are always read from the perspective of their controller, so any text referring to 'you' on the Friend you gain control of is now referring to you, it's controller. Likewise, because you control that Friend it is now 'your Friend' and you would be able to use effects that look for 'your Friends' on it." - dev Adam Hollister (MLPCCG Rules Dev) (MLPCCG Rules Dev) (MLPCCG Rules Dev)
If Pony Charm would cause a player to control multiple copies of a Friend that is Unique, that player must choose one to be dismissed by the game. This can include the Friend with Pony Charm on it.