I have a good addition to TE11 solution (MDKII)

You wrote this:
"you can challenge anyone to a friendly, and only the challenger is affected by injuries, condition burn and skill point gain, while the person challenged gains no drawback or benefit (they only get to see the result of the match)."

And I add- If the other party (the passive) accepts the friendly then he will be as 'guilty' as the challenger (injuries, condition, skill, etc...)