Banks close at 17:00 but their work with the cutomers close at 14:00 if I am not mistaken. Government offices close at 14:00 but around noon they become lazy. Never show up at a government office as early as when they open, they start working for real an hour later. Keep in mind that between 10:00 and 11:00 they will be having coffee and that's also a no no. You can go there but you have to suffer their bad mood.
Yes, there are many Armenians in Lebanon, I consider them Lebanese now as they are third generations. There are still first generation but they are few. I live in an area where Armenians are dominant; nice people.
So your friend is of Armenian origin and is going to get married in the south? (Saida and Sour are in the south)
I have to warn you about something. If you want to surprise him, there is something you have to consider, public transportation. It is is not something you can rely on. You can rent a car but that means you have to learn how to drive the Lebanese way and you HAVE to ask for directions; don't rely on maps
I am sure you will have a great time, September is a nice month.