Your guess is as good as mine Matou. I would be more apt to believe it's the naive side of DM coming through. Could it be he believes he has convinced them and they just go along with what he claims? I'm sure the guards have their ways of "befriending" inmates, gaining their trust to obtain information. It would be something to be a fly on the wall and hear these conversations or talk to those DM says he has convinced. Yes they do plant jail informants and if the information shared is worthy, they would be called as witnesses. MOO.
Maybe if we could get an accurate account of what transpires at Barton St we could possibly comment with ease. If DM is in solitary I doubt an informant would be much use. If he is in with everyone else it should be easy to spot the guy zoning in for a chat with questions about the case.
IMO just because he thinks guards like him, doesn't mean he would speak about the case. If he's not talking to LE why would he talk to someone in jail, other than to maintain his innocence ? JMO