I could compare "current faction" instead of "original faction".
Consequences of that would be:
1. you will not be able to heal a mind-controlled xcom soldier... which (in my personal opinion) is much worse than not being able to heal a mind-controlled alien
2. healing fatal wounds gives experience... which is balanced by soldier being wounded/out of service. Aliens however cannot be "out of service", so the balance would be broken... which is why modders asked me explicitly to implement the feature as it is today.
Original discussion is lost, but feel free to ask e.g. Dioxine.
Am I wrong to think that this behavior should also apply to medi-kits?
There's a lot of places in OXC and OXCE, where current faction is considered and also a lot of places where original faction is considered.
You're not wrong, but you're also not correct.