We use "~죠(지+요)" when we trying to say "~지" politely. And the use of "~지" is coming as below. We use "~지?" when we ask someone assuming that he/she already did it. Second, we use "~지" when we answer to a question with an intension to let him/her to know that we surely did it. For example: "점심식사하셨죠?" "You had lunch, right?" "네, 맛있게 먹었죠." "Yes, I did of course." And finally, we rarely use "~지?" When we tell someone to do something with sarcastic remarks. For example: "점심밥이나 먹지?" "Why don't you just eat(finish) your stupid lunch?"

죠 is contracted from "지요". It is a grammar point. It is informal, as it ends in 요, but still polite. 지요 asks for confirmation. It creates a tag question. It's like adding "right?" on the end of a sentence. "Geez, it's awfully cold tonight, right?" ex. 예쁘죠? It's pretty, right? 이번 주 날씨가 좋죠? The weather this week is nice, right? I'm assuming your Korean friend is female? Tag questions are generally a characteristic of feminine speech. Of course males use them too^^ But since you said your friend is using it to end the majority of his or her sentences, I feel it's even more likely. He or she could be trying to encourage conversation with you too. Ah wait, I see! I'm so unobservant haha. You said "her sentences"...okay, slithering away now :) http://www.talktomeinkorean.com/lessons/level-4-lesson-4/

It could be ",right?" or ",isn't it?".

Yeah, 잖아(요) is like ", as you know."

감사합니다! I've actually used 잖아요 before, but I've never did so with 죠/~지요.