Get a Dremel Moto Tool (tm) or similar and cutting disks (these are typically about one inch in diameter and very thin). Cut slits into the offending nut and/or pipe until these break apart.
Hint: Hold the tool firmly and carefully so the disk cuts a straight slot. Sideways pressure will cause the disk to shatter, although you usually buy the disks by the dozen for quick replacement.
Hint: For cutting plastic, go no more than about five seconds before withdrawing the disk and letting the work cool down. Otherwise the disk will freeze in the slit or shatter here too.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.