The consequences of what can happen on a probation violation are pretty wide and varied. They can range from the agent making a notation of the failed drug test in their file to an outright petition alleging violation of probation. The penalty for violating probation is the same potential maximum penalty one could have received for the original offense; therefore, if the maximum penalty for the assault was 93 days in jail your friend faces up to 93 days in jail.
Unless your friend was certified as an adult, he is under 17 and a juvenile under Texas law. Part of any probation is that the person commit no new criminal offenses, and smoking weed is still against the law in the State of Texas. The probation officer can bring him in front of the judge or handle the violation internally (they call it "administratively" at community supervision). The judge can extend him on probation, order him to do drug classes, to submit to random UAs, revoke him, etc.
Even though it's his first violation, he's only been on probation for one month so I'd think there's at least good chance that he will have to go before the judge. If he wants specific advice, then he needs to talk to a local juvenile law attorney or the attorney who put him on probation.