Having a co-author as an external examiner for doctoral thesis defense I'm a PhD student in Computer Science in the UK. I had co-authored a paper with a Lecturer from another university (not any of my supervisors, just someone's work I'm building upon) in my first year. I have kept him up-to-date with my work and he's very familiar with what I'm doing. Would I be allowed to have him as an examiner for my thesis?

Edit: Just to clarify, I'm referring to an external examiner.
It doesn't matter whether there is a "national or global consensus"; it matters what is the rule at your particular university (consensus means "general agreement"; there may be exceptions).  Given that the concern has been raised, your advisor is exactly right when he says "We'd have to check this."  

It sounds like you don't know who to ask and are trying to circumvent this by instead getting a general answer from the larger academic world.  That's not the way to go here.  If you don't know who to ask, begin by telling your advisor that: this is his problem as well as yours.  Then together you ask various university people about the issue until you get an answer.  If there is some faculty member who has a leadership role in the graduate program in your department (e.g. the "director of graduate studies" or possibly just the department head), start there.  

(Added: Of course you may be able to find the answer to your question in online regulations for your graduate program, and that is an excellent place to start.  I had assumed that the OP would have tried that first before asking here.  Generally posted regulations do not address every eventuality.)

For what it's worth I would be surprised if this were a problem.  But I said what I meant: the previous sentence is not worth much.  Maybe it's a problem at your university: then I'll be surprised (so what?).  The last thing that you want is not to check this out and find out too late that it's actually a problem.