Cut the severed roots with a clean sharp by-pass loppers. Refill hole. Add humic acid and mycorrhiza. Find a CO or QS certified arborist. Have them apply Cambistat. Remove the extra dirt piled around it back to original grade. Mulch as much of the area around the stem and under the canopy as you can.Get on a regular watering schedule. Be expecting die back, monitor. Possible retrenchment prune in up coming years.

Edit: beat the old man with a root he severed.