Depends on how they quit. With the in-game option it couns as a loss. Killing the process from Task Manager, however, circumvents the loss since it's registered as a disconnect.
Apparently even that will be removed, though. Once the next patch hits, you leave the match for any reason or by any means - you lose. Just the way it should be.
Going to be honest, there are some perfectly valid reason's to leave a game in progress, well more so just one.
Fill ins, if you get pulled into a losing game you should be allowed to leave without penalty, Titanfall 2 does this, if you get filled in it alerts you to the fact that leaving will not count as a loss because you've not been here since the start thus your efforts could not turn the tide at this point, but if by some mircile you do win, you get the win bonus, but no, in For Honor, you have to suffer and deal with the garbage that is MM right at this moment, a system Ubisoft themselves barely understands,