Friendly is as friendly does.
The drafting of letters, particularly diplomatic ones, is a serious matter.
“Amicalement”, left in the French, would have been a better choice, especially given the “Cher Barack” salutation, and the fact that unless you are a complete oddball, “Amicably” does not quite cut it in English.
This is one word, everybody, one word.
Context is everything, as translators are fond of saying. The French President should have had a whole department of translators having a meeting on this. Alternatively, His Excellency could have asked what precedents had been set for this type of letter.
In Portuguese, he may have written, “Abraços”, which literally, in English, means “hugs”. That would have been a whole lot worse than “Friendly”.
Two cents, in the spirit of friendship,