If I'm reading your question correctly, your friend is on felony probation for an "assault" of some sort and has now picked up a third DUI misdemeanor.
They're facing up to a year on the DUI, PLUS additional punishment for the felony case. When a person is placed on probation, they basically have the maximum punishment hanging over their head while on probation. If they violate, the judge can impose any or all of the remaining time on the case. Most felony assault cases carry 2, 3 or 4 years in custody, so your friend could be looking at up to 5 years (4 for the assault plus 1 for the new DUI) plus any additional time for any other probation violations for any other cases they're on probation for.
What are his options? Get the best local attorney they can get their hands on.
The above answer is for general information only and is based on the information you posted. Every case is fact dependent, so to get a thorough analysis of your situation, you will need to consult face to face with an attorney licensed to practice in the jurisdiction where the incident took place. Do not conclusively rely on any information posted online when deciding what to do about your case.
It's tough to give a thorough answer without a detailed analysis of both the DUI and the probation case. With that being said, there is a state-wide mandatory minimum of 120 days on a 3rd DUI (within 10 years), and that is independent of any additional jail time that may be imposed on the probation violation. His best options are to hire an experienced attorney to help guide him through this. Good luck.
I agree with the previous attorney's assessment. The period of five (5) years is not unrealistic, depending upon the exact nature of the underlying conviction. I practice in Solano county and would be happy to offer your friend a free consultation about this matter. Good luck.
The answers provided are stellar, and are about as specific as can be given under the circumstances. Without knowing all the facts and consequences of the underlying felony conviction, a concrete number on exposure can not be obtained.