Depending on how close they are he could run cables (ethernet) or fiber. Single mode fiber can go 10km according to some Ubiquiti spec sheets I found on google. Ubiquiti also sells AirMax products that can do PTP or PTMP over the air, although some will be affected by rain. They could even rent space from a radio/cell tower. There are probably a decent amount of other products out there I am only familiar with Ubiquiti.
At this distance you would want a good repeater at about half point instead. Don't forget that data has to travel back and other side might not have such a strong signal