He can justify the drinking all he wants. He drank and no longer has 10 years of continuous sobriety. There's nothing wrong with that and as long as he's honest it is what it is. If he attends AA meetings though and tells people he has 10 years sober he's being dishonest and that's not fair to other people who may actually be 10 years sober. If he's honest and says he had 10 years and decided to drink but didn't like it and now he's back to sobriety that's great and may save others from taking that right hand turn down the road.