Oh boy, a grammar & usage discussion! My time has finally come
One possibility for why you think it sounds weird is because you consider it an absolute adjective and your friend doesn't.
Absolute adjectives (for the purposes of this discussion, please don't @ me with grammar nitpicks) are words that really shouldn't be compared or given a "degree". Unless you're trying to be intentionally cute, an object can't be more perfect or most perfect -- it can only be "perfect" or "imperfect," because perfect has a very specific definition and something can't be only partially perfect...or else it wouldn't be perfect at all. Some other absolute adjectives are false, silent, freezing, dead, and pregnant. Technically, they can only be one specific thing, and they can't become "more" or "less" of that adjective...if they change enough, they simply become not that adjective anymore.
You may have noticed that a lot of these get modified with adverbs anyway. "Very pregnant," "mostly harmless," "Practically Perfect," "mostly dead." Some of this is done intentionally, for humor. Other times, it's just people doing what people do - changing language for emphasis. As truemane mentioned, saying "literally X" is often redundant or just plain incorrect in context, but people do it all the time because language is always evolving.
I'd echo what other people here have said: "forbidden" might be an absolute - something is either forbidden or it's not - but in this situation, the phrase "strictly forbidden" could easily be interpreted to mean "(forbiddance is) strictly enforced".