Illegal sentence? Colorado Sentencing, Plea Bargains

I am trying to determine if my friend, who is in prison, was in fact illegally sentenced. My friend pled to 3 felony counts in 1994, receiving 8 years for each count to run concurrent. It was stipulated in the agreement that the time for each count would run concurrent.
He was released upon reconsideration in 2.5 years.
He was still on probation (not parole) when, 12 years later in 2006 he was charged with another felony. (Incidentally, I know for a fact he was innocent of this felony and was the victim of lies and deceit, and even when he pled he stated in court that he was only taking the plea because he felt he could not win at trial against such odds).
At any rate, he took the plea with the understanding from his lawyer that he could be sentenced to no more that 6 years under Colorado statute.
The judge however, sentenced him to 24 years, running the 8 years given concurrent for the 1994 case consecutive, and gave him 6 years for the 2006 case to run concurrent with the 24 years.
This is a good guy who has a wife and 4 kids who love him and I am intent on helping him so any advice is very welcome.

 

My question was whether or not the sentence was legal.