My friend has just had a spinal decompression. when will it be safe for him to fly?

Depends: Depends on how long ago the surgery was and if he has cleared the normal post op course. Once cleared from a surgical point of view it should pose no problems flying in a pressurized plane.

Yes with the : Yes, once cleared by surgeon, there should be no issues. The clearance is mainly to see that there are no post op issues like wound healing issue or infection, a lung issue including pneumonia or urinary issue including infection. Carrying or lifting baggage may be restricted for several weeks based on extent of surgical incision. If you are talking about a traction treatment, there is no issue.