my nephew is mentaly disabled no one took into consideration his disability first time felony he is innocent but took the plea bargin can he get help?

This is a very complicated area of the law.  I'll try to break it down in the space allotted here as best as I can.

Among other things, to have a valid guilty plea the defendant must be competent to make the decision to plead guilty.  Due process requires that the decision be voluntary and reasonably well-informed.

 Competence and voluntariness are linked.  Plea agreements are deemed to be rational bargains; it is therefore important for courts to satisfy themselves that defendants have exercised free will.  A mentally incompetent defendant or one under the influence of drugs or alcohol is legally unable to enter a voluntary plea.  The standards for mental incompetence are similar to the standards for competence to stand trial; namely, whether the defendant can understand the proceedings and has sufficient present ability to consult with his lawyer with a reasonable degree of rational understanding.

Also to be considered is the question of how well defendants must understand the consequences of pleading.  At a minimum, defendants must be made aware of the main constitutional rights being relinquished.  These include the privilege against self-incrimination, right to trial by jury, and right to confront one's accusers.  Defendants also must understand the nature of the charges against them and the maximum possible sentence.  But when a defendant turns out to have been confused or to have received bad advice regarding such matters as the likelihood of winning at trial or of receiving leniency, courts have not been generous in deeming pleas involuntary.  The opinions weigh the need to provide due process against the system's need for finality in guilty plea judgments.

Let me give you specific examples of factors that may require rejection of a guilty plea.  One example is prosecutorial misconduct.  The courts have rejected guilty pleas induced by such misconduct; something such as a threat or improper inducement  to obtain the plea.  If your nephew was not represented by counsel possibly this is what happened to him.  In such a case, it could be argued that his decision to plead was uninformed.  Another example, if he was represented by counsel where they competent?  The presence of competent defense counsel is relevant to an assessment of both voluntariness and defendant's separate constitutional right to effective assistance of counsel.  As a result, courts hesitate to accept a guilty plea unless competent counsel is present. 

Note:  At a minimum, a court must assure itself that a defendant understands the right to counsel before accepting a waiver of counsel in connection with a plea.

As you can see this can get really quite involved.  You need to speak with an attorney.  And if he was in fact represented in the plea agreement, not that attorney.

Good luck to you.