listen if 5 miles is all he's driven and he's quick a drain of the tank MIGHT, and I repeat MIGHT still save the day.
If there was enough diesel in the fuel pump and line to the injectors he may not have even drawn petrol from the tank.
Twill still need stripping to check, but I DOUBT at 5 miles of steady driving any major damage has been done, If it had it wouldn't run right ?
Massive overcompression detonation etc ?
Not sure about the long term lubricant stripping aspect though...
Either way will be a headache, but a rav 4 diesel refurb should be easy to source ?
Still bad news I'm afraid, but not irretrievable