I assume the international licence was a Croatian one. On its own it has zero validity, only in conjunction with the Croatian one, and then only as a translation if needed. I think it's irrelevant to the issue anyway.
It depends how long he has been in the UK whether his Croatian licence is valid or not - I don't know the exact period but after whatever that is you HAVE to get a UK one. The Croatian one is no longer acceptable
His UK provisional licence would only be OK if he was with a qualified UK driver.
So it sounds like a driving without a valid licence offence.