The specifics of what happened, how it's charged by the DA and the exact charge for the prior strike matter.
Robbery carries 2, 3 or 5 years in state prison. There can be enhancements added for the use of a weapon and any additional crimes committed during the commission of a robbery. The sentencing range is different if it was committed in a residence or at an ATM.
The prior strike doubles any sentence imposed and if the prior strike is a violent offense, then there is an additional 5 year enhancement.
The prior strike can be "stricken" by the court and not imposed, but not the 5 year prior. If there was a firearm used, it potentially adds an additional 10 years that cannot be stricken by the court.
Best case scenario? It was not an armed robbery, the court strikes the prior "strike" and finds unusual circumstances that justify the granting of probation. Worst case scenario? It was an armed robbery with the 10-20-life firearm enhancement and the prior strike is a violent felony. If maxed out, he'd be looking at 5 years, doubled with the strike, plus 5 for the prior plus 10 for the gun for a total of 25 years.
I'd need to see the exact charging document and know what exactly his record is in order to be more accurate. As you can see, the sentence can vary depending on the facts.
This answer is a duplicate of my answer which was submitted in error under a misspelled screen name:
The determination of whether an out-of-state offense actually qualifies as a serious or violent felony ("strike") in California is a complex question. Many theft offenses from other states do not automatically qualify as strikes here in California because some element of the offense is materially different from that here in California. I have recently had success challenging alleged strikes form Arizona, Oregon, and Ohio. If the prior conviction does qualify as a strike, the prosecutor and the court retain the power to dismiss the strike in the "interests of justice." Otherwise, the consequences can be severe: no probation, double the sentence, prison time served at 80%, possible consecutive sentencing, and possible enhancements (five-years for “nickel” priors, etc.). Two prior strikes offenses elevate this new case to a potential life sentence. I would have to have a copy of the complaint to evaluate this matter further.