Marijuana's principle ingredient, THC, remains in trace elements in the lipid cells, because it's not water soluable. But the elements are trace, and they are slowly released over time, though not in quantities enough to get you high again.
To answer your question... the weed itself won't prevent them from weight-lifting. But smoking it is still a health risk, and it's not good for the lungs. And anyone who's lifting weights knows that lung power is very important. Unless your friends wanna start making magic brownies, tell them to butt it out
http://www.bodybuilding.com/fun/berardi43.htm
heres a site that mentions (briefly) that pot lowers T levels, which would not be in the best interest of someone trying to build muscle.