Family Member Reported A Murder Confession.

This is in Indiana, USA.
A couple of months ago, a family member of mine reported that his long time friend had confessed to multiple murders. About a month and a half ago the information he provided police led to two arrests. To my knowledge investigators only followed up with him a couple of times, they got all of the information that he had and were able to corroborate it through physical evidence, cell phone records, etc. He was unsure if he was even going to have to testify at the eventual trial.
Then this week, the post office attempted delivery of two separate certified letters, one from each defendant’s attorney. (He no longer lives at the address where they were sent, so they were both refused.) Is there anything that he should be aware of/careful about? They have no reason to believe that he would willingly assist in either of their defense, is he required to accept or respond to these letters? What reason would the defense attorneys have to contact him? He wasn’t left with any advice from the investigators, and he’s not the sharpest tool in the shed. He definitely can’t afford a lawyer. I’m not concerned that he might find himself in trouble, but he could probably be tricked into saying something damaging to the prosecutions case by the right lawyer.
Thanks, any advice is appreciated.