It won't be an issue unless he tries to flash bootkoader, in which case Odin will stop at Sboot verification.Since You are on 4.4.2, I think, you cant go back to 4.3 since your bootloader is changed. But someguys have done it using mobile odin. search in this forum, I have seen couple of threads for it, but even if you go back to 4.3, you will still have 4.4.2 bootloader, because of which you will not be able to charge your phone,when it's switched off..that is the only issue reported by user's so far I think....
He can still use the ODA tab and ROM only with everything working fine.
And power off charging, its a problem of 4.3 bootloader on KitKat ROM, not the reverse as you mentioned
Fired from my RPG with auto targeting