One of Mike's backfoot nails has grown sideways

It's was not broken, didn't bleed or crack, didn't cause any pain or limping because he is with me all the time.. I don't understand how it happened. We noticed it a few months ago and I blame myself for neglecting it, but we had a lot going on. He is FINALLY getting around (a little bit) to let me trim and file him nails mm by mm. So what can we do to turn the twisted one back? Even though it doesn't cause discomfort I guess it's not good.