I bet it has something to do with how the pipes connect from the evaporator to the outside condenser. How did you make up the connections?
Please add a pic because this is all becoming perfectly clear now. Your tubing is too small. The minimum is 3/8ths tubing for the hi side and 5/8th for suction.
Since he is dealing with very low ambient temps, this system may not operate correctly under 68 degrees outside. He really needs a heat pump capable of defrosting itself.
This system will not operate correctly because the "outdoor temperature" is nearly the same as the "indoor temperature".The unit is so efficient that too much refrigerant is condensing in the condenser. Remember that this is a critical charged unit, therefore the condenser,evaporator,suction line and liquid line will contain the proper amount of refrigerant. If at anytime there is an unbalanced condition, the system will fail to operate in the prescribed manner. The condenser has too much refrigerant due to the low ambient therefore starving the evaporator thus resulting in lower pressure which results in lower temperature (frost accumulation).To remedy this situation, block off the condenser allowing the head pressure to build up to a corresponding pressure of a 95*F day. You will then notice the suction pressure will also rise and now is the time to check the superheat.