If either of the two defendants used a weapon during the offense they can both be held responsible under the law of parties, Texas Penal Code 7.01.
If there is proof of a weapon, even without finding one, then a conviction can stand. As a part of the plea bargain, the defendant makes a judicial confession which typically includes an acknowledgment that a weapon was used. If that was carried out, then there is likely nothing that can be done. I would bring copies of the judgment to an attorney's office and ask for a consultation.
I agree with the earlier answer. Additionally, he took a plea bargain; this means that he admitted to certain facts in order to get the benefit of the bargain.
The only way that anything could be done is to try to reopen the case with a application for writ of habeas corpus IF there is a constitutional violation. If not, then the guilty plea waived any right to complain and failed to put the state to the proof. (That said, it does not matter if anyone was caught with a weapon - if the complainant stated there was a weapon in the hands of either of them then that is sufficient.)