I would check the wiring to the sensor on the back of the wheels he may of left it off/damaged or broken it

Generally speaking discs and pads can be changed without affecting the brake system

 

I would rebleed the brakes,sounds like they might have opened the bleednipples to replace the pads and it all leaked out?.
A common ABS fault is the abs exciter ring on the front driveshafts rust and break,this causes a timing error to the ABS ecu and shows a fault,so something else to check,and as branny said the wiring....
You should look at changing the brake fluid every three to five years for safety,or atleast get the water content of the brake fluid checked,as brake fluid will absorb water moisture making brakes less effective,i.e spongy ,poor effort ,even pulling left or right etc.In extreme braking the heat from the pads can "boil" the water in the brakes ending up in no brakes working...

 

Hi also could have forced the fluid back through both the ABS unit and the master cylinder - damaging the ABS unit and flipping the seals in the master
regards
Steve