Idk about other areas, but in Virginia I don’t believe you have to. I have been training my mare pretty much all by myself and I don’t have any permit. I am pretty sure if they ask you, you can train their horse for them. I would get a written out agreement though, just to make it all in writing so they can’t change the terms on you. I would also advise asking for even a little pay if they are able to, bc training a horse is really hard work!