Its acute effects include stimulation of the metabolic rate. It increases heart rate, resipration, resting blood pressure, and reduces exercise performance. Tolerance to most of these effects develops rapidly, and regular smokers are far less apt to notice them. It also increases appetite of course, but doesn't seem to effect carbohydrate metabolism. Some studies show that smokers consume more calories, but aren't any fatter, than non-smokers, so there is still a lot that needs investigation. Earlier links to gynecomastia have been contradicted by later studies, so there is no universally accepted association.
I assume your question is more focus on whethor or not smoking will help or hinder gains in the gym. I don't think the acute effects are strong enough to really interfere with gains overall for the occassional smoker, and tolerance prevents regular smokers from being as affected by the drug. Although a case can probably be made against it, I think the bigger issue is motivation, which can be a problem for some regular marijuana smokers. If your friend is consistently training, he will probably do fine. If it is interfering with his intensity or motivation to train, it most certainly will hinder growth and performance.
Hope this helps.
