It will depend upon how long his original sentence could have been had he been sentenced to jail when he received Probation instead. That court could reinstate his probation or send him to jail or anything in between.
Consult with a good lawyer in that local area where the case is pending.
If he is on straight or regular probation, then the maximum time he can get is the number of years that were probated. For example, if he got 5 years probation, he could get up to 5 years.
If he was on deferred adjudication probation, then the maximum he can get is the maximum for that offense. For example, if he was on probation for 8 years for a second degree felony, he could get up to 20 years for the probation revocation (because that is the max for a second degree felony.)
The judge could decide not to revoke the probation but to extend it (up to a maximum of 5 years for state jail felony cases and 20 years for other felonies) so that he remains on probation during and after he gets out of state jail.
The judge could decide to terminate the probation unsatisfactorily.
Without more information, I cannot tell you what he is facing but if it is a prison sentence for anything other than a state jail offense, he needs a good lawyer. (If it is for a state jail offense, then the time will simply run concurrently.)
It is not clear from your question what the person is on probation for and whether the probation is straight probation or deferred adjudication.
If he was on straight probation, it depends on what the probation plea was for. For example, on straight probation sentences, there is a maximum time that is part of the plea (3 years in prison, probated for 3 years, etc). In this example, the maximum that he could get would be 3 years, but that amount can be negotiated.
If he was on deferred adjudication, the judge can consider the full range of punishment if his probation is revoked. For example, if someone was on deferred adjudication for a 3rd degree felony theft, the judge could sentence that person to 2-10 years in prison because that is the range of punishment for a 3rd degree felony.
It may be possible to keep the person on probation, you would need to hire a good lawyer to fight the revocation.