Full disclosure: I am a total hardass when it comes to kids for whom (and for the deeds and misdeeds of whom) you are financially and legal responsible. (If he were 18, I'd say tough luck.)
His mother MUST attach - and enforce - consequences for the choice he wants to make.
Perhaps total supervision - to the extent of taking him to and from school, and calling to check in on his attendance every class period - and removal of ALL privileges until he passes three consecutive drug tests. That includes phone, TV, Internet, video games, and school extracurricular activities. NOTHING that is not education-related (and no, sports are NOT education-related).
Remove everything from his room except three pair of jeans, three white shirts, underwear, and the mattress.
Search what's left in the room daily, or more frequently.
"Remove everything from his room except three pair of jeans, three white shirts, underwear, and the mattress."
... and the door.
Perhaps total supervision - to the extent of taking him to and from school,
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Remove him from school!