Thanks, everyone. Interesting to see tha paranoia is present in one or two.
Even more interesting is what you say about some people being able to do no wrong, as that suddenly adds a second explanation to a certain friendship of his. It's basically toxic, I'd go so far as to say he's basically in an abusive relationship with this guy (using 'relationship' in a broad sense). The guy takes him for a mug and has DP running around after him. If it were anyone else DP would have nothing to do with them, but he just can't see it with this guy. He even went as far as to ask a mutual friend why I didn’t like this bloke. He had absolutely no idea.
The tide may be turning, but, as with the threads I read on here from women trying to leave abusive partners, it’s not going to be easy and I'm treading carefully. DP's finally realised that the friend is a user, but when I tried to agree he got defensive of him, so we’re certainly not out of the woods yet. However, if I mention the Asperger's it might help, as he'll realise he has had a reason for being so sucked in.
If anyone has any thoughts on this I’d again be grateful. I can’t get in the way of them too much, as they’d work together so see each other every day. However, other work colleagues have confided in me that they, too, are worried about the situation, so I do have help. Not that any of us are going to interfere other than to try and give DP options of things he can do that don't involve the 'friend'.