Given the following context:

John, an ambitious young police officer, is sent undercover to join a violent football firm associated with the fictitious club Shadwell Town to track down the 'generals' - the shadowy figures who orchestrate the violence. His team of four gradually ingratiate themselves into the lives of The Dogs, the nickname that Shadwell's fans give themselves.  The main site for this is The Rock, a public house around which The Dogs' lives revolve.  Gradually, the hard drinking and hard fighting macho culture - where Saturday's match and Saturday's fight are all that matters - prove strangely irresistible to John and he slowly finds himself becoming one of the thugs he has been sent to entrap.
His relationships with his wife, his superiors and even his team become strained. Eventually his wife returns to her parents' house and rebuffs his attempted reconciliation.
The police operation is abruptly wound up for budgeting reasons, just as it seems John is making progress in identifying those who pull the strings. The closing sequence shows a shaven-headed John taking part in a racist march, having become nothing more than a neo-nazi fascist.  One of his team approaches him to try to help him, but is rebuffed. John says that he is, again, working undercover.  There is a degree of ambiguity, and it might be that he is working undercover, though it may also be that he has become deluded and has merely mired himself in an even less pleasant world.  His fascist chanting at the very end makes it clear that whatever the truth, John is unable to prevent himself from sinking into his character.

answer the following question:

Whose relationship with their wife becomes strained? 

John