If he is just hiding, laying on his side on the bottom, showing a major color change, sometimes a day or two without eating ... those are normal reactions for an O. They get stressed from being moved and like to make us worry a lot. Most Os will do this, even if you just rearrange their tank, clean their tank, strange people visit, loud noises, major light changes. Some are more curious or aggressive and not nearly as much bother them.
Can you describe exactly what is making you worry, his actions, conditions, etc.? It would help greatly for those on here with vast knowledge.
I hope the best for you and your

friend!