I get the worry about new bacteria growing but as for crazy blood born pathogens, its his own blood and he didnt share a rig. So the worst I imagine him getting is some crazy antibiotic resistant strain of common bacteria such as MRSA or something. Although anything IS possible, and it could have some crazy shit on there, HIV/HEP C if thats what the OP is concerned about should not be an issue, considering it is his blood; he is not introducing a different persons blood/infections.
New bacteria? sure. but nothing like hiv/hep
Yes def prob got new bacteria on it. Yes prob a colony of them. Yes it could kill you. Yes, its a VERY BAD IDEA to do that shot.
Plus the bigger issue here is not exactly the new needle. The BIG ISSUE is whats IN the needle and what he would use to get high. If he had a new needle I"m sure he'd try and transfer the solutions, which is just as terrible an idea.
But I KNOW youre going to use it to get high (because the fact that you're even asking this question tells me that):
I wouldn't say its a good idea to plug it by any stretch of the imagination, but if youre gonna do it thats def the way to go.
And as for that bullshit excuse of why not to use a new rig every time, its just that: a bullshit excuse. Get a fresh rig every time man. Its not hard, and its not expensive. Shit, go to a pharmacy. Even if your state has laws against giving out needles to people without diabetes, etc, if you ask enough pharmacies you will find a pharmacist sympathetic and who will sell you one eventually. Just takes effort, something us drug users hate having to give.