For those of you who don't know - I'm on a year abroad as part of my uni degree, so I'm living in Germany working as a classroom assistant in English lessons. The work is really minimal so we have a lot of spare time to go travelling and so on; I've been really lucky with where I've been placed because there are loads of other assistants here and we all get on really well - fantastic.
There are maybe 10 people from my uni course who are also doing this, and two of them are in the same region as me, about forty minutes away from where I am. Both of them are in my group of friends at uni. Another friend from our uni course is maybe 2 hours away, just in the north of another region so she's not so far away either. With the two who are 40 minutes away; one lives in the city and the other one lives just outside of it - and at uni they probably talk to each other the least out of everyone in our group. The one who lives just outside of the city is a really lovely girl, but she's so introverted and off in her own little world that we actually all get really frustrated with her, whenever you talk to her she always asks you to repeat what you said because she wasn't paying attention and she's essentially just a little girl stuck in a woman's body - she lives at home and is a very family oriented person, so for her to move to another country is quite a big deal, and I think she's very homesick.
However, the four of us were at a training course in early September before we went our separate ways, and she was off in her own world, clearly homesick (kept going off to her laptop whenever we had a break to go Skype with her family) but wouldn't admit it (and it's not like anyone was going to make fun of her for it, none of us were delighted to have left home) and was bringing the mood down with her sadness, the three of us were trying to have a laugh before we went our separate ways and she was ruining it because she was upset/sad/whatever - obviously we weren't doing things without her, but we were wishing she wasn't there so we could have fun instead of being brought back to reality by her sadness.
The point of all of this: last week I went to the city that my friend lives in to hang out with her for a few hours, and we didn't tell the other girl I was going to be there. It was a fairly spontaneous decision - I finished work at lunch time and the girl in the city had the day off so I just hopped on the train (I have a free travel pass, I should mention, that works across the region - so I can go wherever I want, whenever I want) and we hung out. The previous weekend; the two girls had gone to visit the friend who lives 2 hours away for the day; and then they went back to the city but the friend who lives in the city then got the train back to visit our friend who lives 2 hours away and spent the rest of the weekend with her without telling the other girl - because, once again, she was off in her own world (they went to the zoo and she kept wandering off by herself) and made the other two feel awkward because they felt like they were babysitting her, not hanging out with friends. So I went to the city and we didn't tell her because neither of us could be bothered with her. I didn't feel bad about it, but she then found out because I put a couple of pictures up on Facebook and took it badly - posting a passive aggressive Facebook status about how "people can be ***** sometimes" and posting stuff like "I don't fit in anywhere" - should I feel bad for what the three of us have done/not done or do you think it's okay for me to do what I've been doing? Normally I do my best to include everyone and I don't think I'm a nasty person, but I feel as if the circumstances have turned me into a bad person - should I be including her regardless of her attitude when she hangs out with us or should I not care? We're only here for 9 months and I feel like I should be making the most of it, not looking after people who aren't enjoying it as much as I am. I'm feeling a bit confused and would appreciate some input from other people. I hope this all makes sense, I've not used names or locations because I don't want any of my pupils googling me and stumbling across my online activities

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