<human>: Give me the gist of the text in a nutshell

We both started the company at the same time. We are pretty close to each other, always joking around. We eat lunch together, we usually leave together and interact a lot during our course of work although we work for different boss (just sit close to each other).

She got an offer for a position at another firm in another city that will be a major step up in her career. She had to give them an answer by tomorrow and she sought my advice today. I thought she would stick around at least for the busy season, so I told her to definitely go for it because it was such a good position. Only later she tells me that she took the offer and this will be her last week because she has to start this new position ASAP.

I never thought I really liked her, thought of her more as "that girl" from work who is like you're best buddy of a sort, kind of like Jim and Pam but with less collaboration and more crap talking. I have been casually dating another girl from even before I had known her, and I never thought of ruining my career at this prestigious firm I worked my ass off to get into by dating someone from work.

But I have been so upset since she told me about getting an offer. If this was a guy friend who was close to me leaving the company, I would just taken him out for drinks and have some bro moment and that'll be all. I don't know how to say goodbye to this girl because I feel so upset right now. On one side I want to not do anything and focusing on getting used to working without her around in the office. On another, I want to take her out to dinner or something, but I don't want there to be a romantic notion - because a romantic farewell dinner is the last thing I need (besides, she might think its weird).
<bot>: Close female co-worker is leaving the firm this week and I am very upset. I don't know how I should feel about her leaving, and I don't know how I should tell her goodbye.