Eric, it doesn't matter because you're not using "friends" as a noun - "being friends" is actually being used here as a corrupted verb, in a colloquialism, much the same as you get corrupted adjectives nowadays like "bendy" "stretchy", all of which are basically being lazy, instead of seeking the correct terminology. However, and it's a big however, if you are writing for effect, and I can see you are, you can get away with colloquialisms like the above, and incomplete sentences like "She used to be." Technically, grammatically, that statement and the next - "Then Tracy got to be friends etc" actually together only make one complete sentence. Do you see my point? Language is evolving continuously, so don't lose any sleep over "got to be friends". If you ever get to the point where a line editor is working on your text, you will have many time wasting moments like this, depending on that particular editor's foibles, i.e, what matters garmmatically to them. And guess what? They're all different. My current editor (and very good she is too) hates the word "then", which looks to me as if that puts you on a loser straight away!! ;-)) My suggestion would be to concentrate more on saying what you mean. Worry about the grammar later. And talking about "then". A very weak word (so my editior keeps telling me). Try substituting "Before".
http://chrisscottwilson.co.uk