What happens regarding the First Offender probation has a lot to do with how his PO feels about things and how he handles this new charge. He can be violated whether or not he pleads guilty to the charge, so he needs to be very careful.
How much time gets revoked and whether you get to keep First Offender Status will depend on what the probation officer is asking for and how the judge reacts to the facts. If his felony conviction involved drugs that will add to the fire. I would advise talking to some local counsel and local public defenders to see how the judge has reacted in the past (I would not advise doing it on this website). It sounds like a lawyer is needed in both cases. Good luck.