we see this all the time - the reason that mortar joint has opened is due to lateral exterior soil pressure causing the wall to lean - IN,,, since the wall isn't flexible, it cracks.
typically, the repair is placing a steel beam vertically to prevent further movement but, as the top of the beam is normally attached to the floor joists, that just transfers the force to your main floor ( never like this method ! ) look on our w/site for an engineered strengthening system which will be invisible,,, its a chicago pro engineer design from about 30yrs ago as far as i know,,, if you can't appreciate/justify the contractgor's price ( cheap to me ), renting a professional engineer ( $ 500 ) will knock your socks off,,, there's also exterior work that we do using this system.
your home inspector is a dufus as are most who buy computer word packages,,, IF you want the work inspected/evaluated/diagnosed properly, retain someone ( like dan ) who can go into court w/accepted bona fides,,, this damage MIGHT have occurred when the home's walls were laid but i doubt it,,, to prove this point, lay a 4' level up onto the crack & measure the wobble - do you think this house would have sold w/that defect when new ?
