I agree that partial takeover has got to be very close to statistically impossible for the reasons mentioned above.
A full takeover is very possible.

Was your antenna down at the time and his fully extended? If so his signal could easily be a lot stronger than yours at your RX.
Especially if your RX tuning may be a little off one way and his TX may be a little off the other way.
When you did the test on a stand, was your TX on? Antenna up?
He would have to have been flying a Futaba since none of the different brands of PCM will talk to another.