I know this is a controversial topic so I fully expect very mixed reactions.
In my opinion.
There should be no need to reinforce someone not part of your kingdom or your kingdoms confederates.
This includes spiking.
"But my friend and I are in different kingdoms."
Well then how about you use the relocate function, you know, like it was intended for?
"But we did fast settle and are not yet in any kingdom borders."
Make use of the duke invite mechanic.
"I want to be able to see who are farming."
You got 2 nice tools for that: Top 10 and scouts.
"But I don't want to risk all those scouts"
Oh no, sounds like you gotta weigh the risks. Are we gonna make the game skill based now? How terrible.
Reinforcing non-allied players can only be used for unsportsmanlike things.
This persons situation could have been avoided by not allowing reinforcement of non-allied players: Hacking
It would also fix spiking.
Travian Legends already has this. From what I understood most players really like the mechanic. Mainly cause it helped prevent personal def multis and spikers.