Basically lets say that he gets a CIMT conviction, which is what an indecency charge would most likely be. He gets convincted, he would be placed into removal proceedings as he was never admitted (and instead got a deferred inspection) and he's now inadmissible, meaning that the government cannot admit him into the country meaning that they are seeking to deport him.

However since during all this time he remains an applicant for admission, the court gets to review whether or not he is admissible and it gets to entertain any waivers he might be eligible for, effectively the court becomes the place where he gets his deferred inspection. This is the point you'd file a CIMT inadmissibility waiver (which has to be associated with an application for an immigrant visa, adjustment of status, or admission into US and keep in mind he's an applicant for admission), in order to waive the ground that's making him inadmissible, if the court approves his then he gets legally admitted into the US and gets his green card back.

Definitely point this out to your lawyer, some might not know you can use an I-601 this way, the lawyer I worked under didn't and only thing that saved that client was my legal research and creative thinking.

However save the CIMT waiver for when getting anything better in criminal court fails. First he should try to get the charges dismissed. Then he should try to plead down to a lesser offense that won't be a CIMT, opposing counsel is generally willing to plead a first charge all the way down to a disorderly conduct charge especially if the evidence isn't particularly solid, if that fails take it to trial and seek a non-guilty verdict.