I've had a similar experience with an aged relative and I know exactly what you're going through so big hugs and buckets of sympathy from me to you and your pa.
Of course YANBU - these scumbags deserve everything that's coming to them when the law of cause and effect eventually comes into play, In the meantime relieve your angst with Dirty Harry type scenarios 'you feeling lucky, punk?' while you're holding a Uzi.
Does your pa normally leave the key in the door or did he forget to remove it? If he has any memory impairment it may be that someone needs to check last thing that his home is secure, but I doubt that will be enough to reassure you at the moment.
Perhaps this type of gadget could be of use [http://www.c-p-p.co.uk/product/asp/ProdID/2363/CtgID/1031/af/page.htm]] or install a couple of dummy CCTV cameras (cheap as chips on eBay) where they can't fail to be spotted (most of them have flashing lights and look convincing).
A full on burglar alarm might not be practical for your pa, but you could consider installing a dummy alarm box (also available on eBay) in a prominent place.
Opportunistic dirtbags have been known to return to the scenes of their crimes but if you've taken steps to install some visible deterrents it may make them think twice about chancing their luck again. Maybe dawn to dusk sensors or time-control switches light sensors give the appearance of someone being up and about?
As you live next door could you rig up some kind of listening device (baby monitor type unit) for your added peace of mind?
Have the police been any help? Ask your local Crime Prevention Officer for useful advice on security.