How to gain weight?

[blahaha]

My friend went on a 3 or 4 day meth binge two weeks ago. He's done it about ten times in total, but never without a month or two in between. Anyways, this last time he did it, he went from 160 lbs to 145 lbs. Ever since then he's been pretty lazy, he's eating more junk food and stuff, and has stopped working out. He probably needs to work out more, but isn't too out of shape. Anyways, he can't seem to gain that weight back! Any tips? And should he stop working out until he gets that weight back or go back to working out every other day? Oh yeah he's 18 years old. Thanks.