RE: Futaba 608FS reciever locout/failure due to heat
How much heat does a power expander generate?
Were the antennas outside of the composite fuse? (some composite materials and carbon fibre block the signal)
Was the receiver shielded in any way? (2.4 components are UV sensitive)
Too many variables here but the easiest is to raise the beaten to death temperature issue. If the receiver got that hot that quick it may be a defective one or something else caused the rapid heat build up or signal loss.
There are numerous positive reports of these receivers working fine in high heat environments to blame the entire series so your friend may have a defective one or an installation issue.
And for all the experience you listed, why was a range check not performed and why was throttle failsafe not set?? (This is safety 101 for gassers).
In this case the receiver could be defective or there was an accumulation of oversight leading up to the incident. As an A&P you know that most air crashes are an accumulation of errors, few of which are equipment failures.
My opinion is we are not getting the whole story here.