Without knowing the facts, there is no way to give you a meaningful answer to this question.
Most robberies carry a maximum term of five years in prison, but other factors -- using a firearm, committing robbery in a home or at an ATM, or doing it for a gang, just to name a few -- can increase the potential sentence. I have no way of knowing how they got to the 28 year figure.
If your friend actually helped the robber commit the crime, the law considers him just as guilty as if he had committed the crime himself. If he helped the robber escape capture after the crime was committed, he would be guilty of Penal Code 32, accessory after the fact, and would be facing a maximum of three years in prison.
In addition to any time he gets in the new case, he could get anything up to the maximum for the crime for which he is on probation, minus credit for time already served.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
Something about the math isn't adding up. As Mr. Marshall pointed out, an accessory (somebody that does something after the crime has been committed to help another person escape, hide evidence, etc) faces a maximum of 3 years in state prison.
If your friend was accused of participating in the robbery (either doing it or 'aiding and abetting'), then they face up to 5 years prison for most robberies.
There can be sentencing enhancements based on a prior record (strikes double the sentence, for example) and the use of weapons (and those matter whether your friend actually had a weapon or just another co-participant), infliction of injury or if there were multiple victims.
Sentences can add up quickly, but it all depends on his prior record, the specific charges filed against your friend and what they can actually prove.
there is no way to answer such a question, primarily because the charge you plead guilty to or are found guilty of is what will determine sentencing and not the crime you are intially charged with... furthermore, there are many factors that lead to the outcome of a case, these are all things that can not be analyzed in this medium.