If the prisoner is guilty, it may or may not matter if he worked on his own. What Plaintiff said could ONLY be untrue, if the prisoner was guilty and worked alone. So the solicitor's claim would immediately implicate that this is actually the case. That's not really helping his client, if you ask me
The original statement implied a false causal relationship: that guilt in that case required an accomplice. The statement is untrue because no such relationship exists. It matters not whether the defendent is actually guilty or not.
"If the sky is blue, then you are a raspberry."
That statement is untrue, EVEN IF you actually are a raspberry. The reason it is untrue is because it implies a false causal relationship. The color of the sky cannot cause you to be a particular type of fruit.