Unfortunately, it could be worse than what you describe. Since he is a first offender, he could be resentenced to the maximum. If it is just one count of felony possession of marijuana, he could get resentenced to 10 years minus the time he successfully served on probation. Hire a lawyer ASAP.
This student definitely needs a lawyer. A new violation can result in a minimum of two years revoked if the new offense is a misdemeanor, the balance of his probation if the new offense is a felony, and, since it is first offender, the court can not only rescind the first offender status, but resentence him to up to the maximum sentence on his original charge. The time he spent in jail in another county on the new charges generally won't count for the revocation case. None of these results are absolutes, but when facing the possibility of prison time, he's going to want (and need) an attorney to get him the best possible outcome.