"Really confused at the posters who think a man should be shoving round a woman who may have dementia." why is it that every time someone does something completely unreasonable such as, oh, I don’t know, force entry into someone’s house, there is always one who insists that they either are mentally ill or have dementia. Presumably even if that were possible the individual would have come alone.
As for OP’s DP, I do think it’s possible he was caught unawares, having let the woman in he was standing at the door speaking to the man, and while many of us might have been suspicious, the man’s attitude may have put him at ease after the woman’s unreasonable behaviour and the fact that she really did then rush to the loo when she came in.
I do think it’s still possible it could have been a distraction burglary, but we are all capable of falling fowl of a scam, we just have differing thresholds, which is why there are so many different, and successful scams.
And his insistence that he did nothing wrong is quite possibly because he doesn’t want to admit to himself as much as anything, how stupid he really was.
I am about as intolerant as they get, and usually answer the door to strangers with a “what do you want!” 
but a few years ago I answered the door to someone who started rabbiting on about something which tbh I can’t remember what it was. I had shut the dogs away when I opened the door (mine, and a friend’s dog I was looking after who is a huge GSD-cross,) and I could hear them both barking non stop in the lounge which is on the back of the house. It was only when I shut the door that it occurred to me that it could have been a distraction burglary where the bloke at the front was keeping me occupied while someone went round the back.
I’ll never know whether it was. But if it was meant to be that, I think the sight of two dogs, one of which was a GSD cross so probably has the capacity to look quite vicious and certainly sounded it, was a pretty fair reassurance that they wouldn’t be back. .