On lumber under 15% MC, the two meters should be within 1% MC if used correctly.
The pinless meter is very sensitive to density; the most common reason for poor readings is that the incorrect density for the wood being tested has been entered. Note that this meter reads an average.
The pin meter reads at a spot--the tip of the needles with insulated needles or the wettest spot for uninsulated needles, but it is just a spot. Temperature corrections are needed.
So, comparing a Wagner to a pin may not always give the same values because one is the MC at a spot and the other is an average. So, the two meters may disagree if there is a moisture gradient within the wood, if the density is not "normal" for the species, or the wood is not between 60 to 80 degrees F.
Wagner used to sell a calibration plate for their meters that read a specified value (like 10.0% MC, depending on the model). Delmhorst used to sell a calibration resistor (20 megohms, I believe, that should read 20.0% MC). The Delmhorst resistor is now built in to their meters, so calibration is automatic. But, it did not matter whose made the meter; 20 megohms should read 20% MC for all.
I have seen static electricity fool a meter. I have seen temporary (several hours) condensation on a cold meter brought into a humid location goof things up. Weak batteries are also an issue.
Was your friend's meter made in USA? If not, it is common to find non-USA meters with faulty calibrations.
If you have a "Made in USA" meter, you can send it back to the factory for calibration, cleaning and checking. If it is critical, at least one company will send you a loaner while yours is in transit and being fixed.
I compared the three most common meters (Delmhorst, Lignomat and Wagner) on kiln dried lumber at room temperature with no MC gradient. You can see the results in the Appendix of DRYING HARDWOOD LUMBER, page 130. This book is in the Archives here at WoodWeb or on line.
OK?