There have been multiple studies that have linked Prepubescent teenage marijuana usage with stunted growth.
Citation 1: Dr Syed Shakeel Raza Rizvi, of the Agriculture University Rawalpindi in Pakistan, says " Marijuana use may provoke a stress response that stimulates onset of puberty but suppresses growth rate..."
Citation 2: "A recently published study" shows " ...In addition, both males and females who started regular marijuana use prior to age 15 were physically smaller in height and weight; this effect was most dramatic in the males."
My only issue with this source is that they don't provide information about the study, they only reference it. They don't provide you with a way to view the results, the only tell you.
Citation 3: Although this study is referring to Tobacco smoking, a few points are still valid when it comes to marijuana.
"Smoking will not affect the pituitary glands which produce the hormones necessary for growth, but however it does result in an increase in carbon monoxide in the blood. This then, in turn, means that the blood has a lower concentration of oxygen and of the other things that it needs to build and repair tissue such as minerals and vitamins. What this then means is that when an area needs protein or other nutrients to grow, it will not be supplied with them as quickly or in such high quantities and quality. This may then mean that that function of the body is not carried out at all, or just that it is not carried out well."
If you friend is smoking blunts (Tobacco) #3 is even more valid.
Unfortunately, due in part to federal bullshit labeling of Marijuana as a Schedule 1 substance, many schools, and medical facilities haven't done too much research to be more certain of the effects.
As a side note for your friend, remind him that in college and beyond (NBA) drug testing before games will occur. Even if federal regulation changes, sports is unlikely to change as many players have said Marijuana has "helped" them and the whole point is "No performance enhancing"
I'll try to link to more firm studies, but it's hard to find them due to what I said before.