How did French people greet in 1400?
In particular I'm looking for french expressions that a peasant like Saint Joan of Arc could have used to greet a friend.
You could use one of the following expressions :
[Hé,] Que y'a ?
Ayez un bon jour
Literally "Have a good day", used to say "Hello" (became "bonjour").
Diex vos sait (very formal)/Diex beneï toi
"God bless you", very ancient, used to start or end a conversation.
You can find more information in « Quelques formules de salutation en ancien français » (Anker Teilgàrd Laugesen, in Revue Romane, Vol. 8, 1973, 1-2).
There is a possibility that many used the traditional Faire la bise, which means to give a kiss. This is not just the ordinary peck on the cheek, it is the double cheek kiss that we often see on older foreign films. It is a common way for the French to say their goodbyes and greetings. It is in no way used as a romantic display of affection.
I hope this helped!