Is there a possibility of appeal after being sentenced if you plead guilty due to bad legal representation?

Well that is a sticky situation.  First, I have no knowledge of the crime he was facing, the plea deal, and the evidence against him.

That said, most states allow a defendant to withdraw his guilty plea if it was made under duress or some uniformed reason.  This is usually an uphill battle because he waives these rights usually in his written plea and before the judge.

The other avenue, would be to appeal the effectiveness of counsel.  That means, the defendant, or a new attorney has to file an appeal and allege that defendant recieved ineffective assisstance of counsel.

Hope this helps.