Can my friend reverse his plea? What is all involved?

A friend of mine lives in a house where a probation search on another tenant (who is a probationer) was conducted. Before the PO's and police went inside, it was clearly explained to them about what is common area and what is the probationer's area, and what is private area. There were many witnesses to this. When the police went inside, it was as if they had not heard a single word that was said. The police opened all doors, popping their heads into private rooms. My friend was in the room and there was a paraphernalia device on the bed. He felt obligated to say it was his and not get the homeowner in trouble whom it actually belonged to. They then searched the entire room and found a very small usable amount ($10) in her dresser. He claimed this as well and is being charged with possession of a controlled substance. The Public Defender told him that if he did not plead guilty, he could be looking at 6 years or more if found guilty. He believes he was railroaded into pleading guilty for prop 36 instead of not guilty and taking a chance at such a meager amount that was gotten from conducting their search in a known private area. They claimed to understand very clearly just moments before they totally ignored the limitations of the warrant. They were even told they were going into private areas while they were doing it. They just sort of smiled and stuck their heads in anyway. There were many witnesses to this also. I have been told that they will just lie and say they were given permission to fully search, but their were at least 3 witnesses that will say the truth of what really happened. Does my friend have and recourse in reversing his plea and claiming not guilty since the search was highly illegal?