Eeks, cover-up is a bad option ... one lie leads to another to another and on and on. I agree with friedbeef that honesty is best .... and it may even be an advantage for your friend because he can talk about how he overcame the odds to straighten himself out mentally .. that I think is really positive!

Look, if he gets rejected because of that, it will be a blessing in disguise cos only professional and mature bosses or companies will be able to recognise the strengths and abilities of your friend.