This is research for you... I have found the Luther episode you were watching, the link is here:
BBC - BBC One Programmes - Luther: Series 2
The converstation starts at about 22:10 minutes in.
The characters definitely say 'disco', I think the woman is using the word as slang, to suggest that the detective is rather unorthodox or unconventional.
I think she is using the term 'disco' like 'mad' is sometimes used in slang, (not meaning someome is actually insane, but rather they are a bit 'out there'). Sorry I can't explain this clearer.
The second time 'disco' is used, is when the friend enters and only hears the girl calling the man 'disco'. I think the friend is just going along with the girl, jokingly calling the man 'disco' even though he probably doesn't know what it means.
The last time 'disco' is used is when the friend says, 'let's disco'. I take this to have a slightly different meaning, this time 'disco' means, 'let's go'. Like someone would say, 'let's do this!', or, 'let's rock' when they are about to do something fun or possibly dangerous.
Like I said at the beginning, I'm not a teacher only a native. I hope this helps though.