Remember talking about squatters rights while getting my degree. Called it out for bullshit then, and everyone agreed. They exist, as someone has said, from common law to allow for unused property to become used again. It has to be unoccupied for either 7 or 12 years, forget now as it keeps changing from one to the other, but it can then be legally transfered without consideration (money) paid to the owner. But it is something of an out of date law (Magna Carta 1215) and recent changes have made that virtually impossible to happen.
If memory serves, and it was quite a few years ago, his property rights remain absolutely intact. They can't sell or change his house without sending notice to him that they intend to, and he has X months to respond before they are allowed.
Also, if memory serves, he is allowed to use "reasonable force" to reclaim his property. So essentially he has 2 options, legally speaking. He can a) go in and kick their asses, and throw them out or b) wait for the court to get around to doing it for him, and then charge him for it.
Oh, and as a quick side note, this isn't just English law that allows for squatters rights. Any legal system based on English law (so pretty much all western ones) will likely have something like this. US law certainly does.