It wouldn't just be the family knowing that he's a correction officer and the potential risks there. These jobs often come with stringent conflict of interest policies. My guess is that he could face seriously disciplinary action or worse if he did it.

its not hard especially if known gang memebers were recently released and invited. so much bad could come from this. its a big risk not only for himself but for the family as well, especially the groom.

I'm confused to as to why her family would keep you from being best man. can you not have any recreational contact with known criminals? I'm asking because I'm truly ignorant and sont want to assume.

The best friend's new wife's family having criminal ties is a problem for a few reasons. 1: Conflict of interest- the management of the prison would very likely be concerned about being on friendly terms with people who may wind up in the system, are in the system, interacting with people who further commit infractions in the prison itself, or people who know people in the above situations. Management may be concerned OP may feel pressured to do the family favors, be coerced into them, or otherwise have their integrity as a correctional officer potentially breached. 2: Rival gangs- yes, being associated with a large prison gang may make OP somewhat safer them THAT gang, but it would also make him a target to rival gangs. Even if the gang makes up an overwhelming majority of the prison and tries to keep them under their protection, it would only take a few minutes if not seconds for OP to wind up a victim to gang rivalries they aren't even really part of. 3: If OP witnesses a crime or overheard details of a crime, or the details something that would be a violation of their contract and doesn't report it, they could lose their job, paying legal fees, and even wind up serving time themself. 4: Guilt by association- Even if OP keeps their nose clean and does the social dance perfectly, if people hear they are associated with a gang, even in a distant way, they would likely color them as being part of the gang. Other officers may considered them crooked and either hound them, stick them with the harder work and hours, or try to stick them with dirty tasks that are against regulations. Members of other gangs would possibly treat OP's every action as being because he is part of the gang, which goes back to my second point. There are probably more, but the above are already good enough reasons

Because it looks bad, so bad that he might be accused of being unfit of his job as a correctional officer.

lol their are many reasons why that could go awful. if anything decline the wedding, explaining why it a bad idea and when they get back from honeymoon take them somewhere nice to apologize or whatever lol.

Except he wouldn't be part of the family at all, just best friends with a new addition to the family

yes