Personally I would have been happy to help the police with their inquiries and to prove my innocents, what good can come from threatening this man with violence, I think he had probably been through quite enough after being burgled.
I wouldn't have any problem with being named as one of the recent visitors to the property so long as the guy wasn't saying I was guilty. Indeed, although I'm not aware of any recent incidents, I can think of four of my customers that have been burgled over the years. Two of them told me that one of the questions ased by the police were "Have you got a window cleaner?" Both said yes but also said I had been going there years and that they didn't suspect me. One of the other two phoned me up to ask if I had noticed a broken window at the back - in spite of only being required to to the front and side windows due to poor access. When I told her that, she said that she would pay me "anyway" - a word that led me to believe that she didn't trust me. I had only cleaned there once and didn't do so again. I did continue cleaning for the next door neighbour though (who was a policeman). The policeman eventually cheated me out of my money so he got the chop too. The remaining one was actually employing two window cleaners which I found odd. I went there twice in all because she was burgled between my visits and clearly had a problem about something or other.
Anyway, I digress.
My point is that I would be very unhappy about giving the police a DNA sample. Although I would be tempted to supply it to get myself ruled out, the fact is that the police keep these samples on their records forever - whether innocent or guilty. Although it would be a simple matter of arresting me to obtain the sample, I would not give it up voluntarily. At one time the police were only supposed to keep DNA samples from the convicted. Then they were only allowed to keep them from the charged. Now they are allowed to keep them from the arrested. Did you know that a while back the list of arrestable offences was drastically widened? You can quite literally be arrested now for a motor speeding offence. Legally they can keep a DNA sample from you for being a few MPH over the speed limit (provided they arrest you). Now I realise that the "nothing to fear if you've nothing to hide" brigade will want to shout me down on this one but I, for one, will insist on them going through the process if they want a sample. It's no problem for them. They only have to arrest me for a short while and do a bit of paperwork. They're going to keep the sample forever anyway so they should at least have to work for it.
Also, as the database grows and the weight attached to DNA "evidence" increases, imagine the possibilities for planting someone else's DNA at a crime scene. Although most coppers wouldn't do it, there will always be a few dodgy ones who would jump at the chance to get someone off the street. It doesn't even have to be coppers doing it because villains could stitch up other villains.
I know I've widened this thread a lot but if the customer was pushing the cops to take his DNA then he is out of order.