That's exactly what i was coming here to say! Dude's just a piece of shit. If he was so deep into adrung addiction that he couldn't control anthing he did and was fucking someone outside of his relationship as a result then there's like zero chance he would be able to go 2 weeks without getting high. I bet he's just using the drug thing as an excuse to be a shit person. 
What drugs could he possibly be doing that he can just take a couple weeks off? This guy fucking someone for weed or something? Come on. Whole thing sounds like bull shit to me. He's just a cheater.