In murder prosecution where law of parties applies. Can the prosecution allude to the fact that the Codef has pled guilty? When can the State do this? Any help is appreciated.

Might want to review the Sixth Amendment first. And a little case called Crawford.

In other words, no.

I't a live witness, as in the Co Def

No Crawford problem, then. And it's going to come out anyway because it's impeachment evidence (both because it's a conviction for a felony and for bias issues as well). Be prepared for the onslaught of questions by defense counsel about whether the plea agreement required him to testify favorably to the state.

How does the 5th apply? Is co-def willing to testify? I just had co-def take 5th after his jury conviction in trial even though he originally told both his counsel and def's counsel he would testify. Does it matter that co-def entered a plea? Was it a plea bargain with agreement to testify?

If the co defendant has already pled in his case, he waived his 5th amendment privilege to do so (jeopardy has attached so the constitutional protection is inapplicable to him now - there is no danger from the government of another prosecution on this issue). Unless there is a case out there of which I am unaware, he cannot now assert his right. Two caveats: if he preserved his right to appeal his case during his plea (instead of waiving appeal), he may still be able to assert the 5th, or if he is asked other questions that might still subject him to another prosecution (e.g., different uncharged offense unaddressed by the plea).

My guess is that he doesn't want to be the only one hung out to dry on the charges and will testify anyway, seeing as how it appears he's a state's witness.

He can still assert his 5th if the case is on appeal, as it is still pending.