"Tu quoque, Brutus, mi fili?" Grammar question

I suspect that your friend misremembered the phrase.

Concerning the actual phrase that Julius Caesar said, the biographers offer conflicting evidence. Suetonius tells us that Caesar died in silence, though he admits the tradition that Caesar said in Greek: "καὶ σὺ τέκνον;" Cassius Dio echoes Suetonius, claiming that the "truest account" says that Caesar was silent although some claim he said the above Greek phrase. Shakespeare famously gives Caesar the words, "Et tu, Brute?" Perhaps he felt that Latin spoken in an English play would have a similar effect to Greek spoken in a Latin context.

Regarding the particular phrase you cite ("Tu quoque, Brutus, mi fili?"), I suspect that it is misquoted, since it returns pretty much 0 results in a web search and it's not good Latin. As you note, the vocative of "Brutus" is "Brute." "Fili mi," however, actually is the correct vocative.

Modified in this way, the phrase "Tu quoque, Brute, mi fili?" does appear to have some currency. For reasons that are utterly opaque to me (if Wikipedia is to be trusted), it appears that different language traditions favor certain versions of Caesar's final words, e.g.:

• Italian: "Tu quoque, Brute, fili mi?"
• French: "Tu quoque mi fili?"
• The rest: "Et tu, Brute?"

Despite the title of the Italian article, however, I can find no source that actually ascribes these words (or their Greek equivalent) to Caesar. It seems to be an explanatory gloss.

To summarize: Caesar probably said nothing when he died, but historians mention the account that he said in Greek: "καὶ σὺ τέκνον;" "τέκνον" can mean "child," "young man," or "son," so "fili mi" is a reasonable Latin translation, yielding "Tu quoque, fili mi" or (most literally) "Et tu, fili?" There's no evidence I can find, beyond Shakespeare's dramatization of the incident, that he addressed Brutus by his name.

It should indeed be Brute, not Brutus, and the vocative form seems to be far more common if you make an internet search. The person who told that the last words came with Brutus appears to be slightly misinformed, perhaps due to knowing that the name is Brutus but being unaware of the Latin vocative case.

The nominative Brutus would make sense if it was the subject of some clause instead of just the addressee in an explamation, so you would need to supply something like me cecidisti ("[you] murdered me") to make it work. But even then I would take tu as the nominative and put the name in the vocative.

The nominative of "my son" is meus filius but the vocative is mi fili. Words ending in -eus and -ius tend to get a shorter vocative form -i rather than -ee or -ie. This does not apply to all nouns ending this way, but it is common.

Different sources give different last words for Caesar. I will not discuss the whether he actually said so, so take this only as an analysis of this particular quote attributed to him.