I try to fight in a multiplayer Battle with a Friend, when she accepted we can't fight because it appears :It seems someone already ended this battle! So, we are imprisoned in this fight. How I do to leave it ?

Here Click 'Get unstuck from battle', hopefully that will work.

Multiplayer battle is broken at the moment and has been broken for a while, And probably will stay broken until the new Battle coliseum comes. To fix this, File a Ticket explaining what happened and the Admin's will unstick you ;)

Thanks ! I will try the Ticket.