That isn't the first time we have heard about something like this happening: courts, DAs, cops, IRS etc. are all using the internet now to find information on those they have cases with.
That said: your friend needs to consult with an attorney regarding the probation issue. The reality is if they don't tell the PO and the PO finds out eventually then the damage could be worse. If they are still sitting in jail (since you are trying to decide whether to post bail) then you can pretty much figure that the PO will find out. Most probation requirements include informing the PO when the person gets in more trouble.
What could happen? They can violate them and they would have to serve all of the original sentence. Judges are not happy campers when probationers ignore the requirements of the court. Bare minimum they can be placed back on probation.
As to posting bail: if you post bail with the court the only way you would forfeit the money is if your friend jumps bail. Given that they can't seem to stay out of trouble my personal opinion is that this is a risky financial adventure for YOU. If your friend can't afford to post bail then they need to contact family or wait until a court hearing. I know you want to help your friend but it may be that the best help they can get is to stay in jail and learn a lesson.
"That's just my opinion, then again I might be wrong." Dennis Miller