For all those saying this is a learning curve and she should be able to go round a foreign capital on her own, please have some empathy. I know on MN the second someone turns 18 they have to be tough and with it, but this is a girl who has just finished college, is on a trip without family, is upset because her travel companion has upset her and left her, and she is alone in a place where she doesn't speak the language.
I feel for her. Yes, she can go and explore but have some empathy!
I do feel bad she has fallen out with her friend and that would have dimmed the trip somewhat but op has made her dd sound hopeless and lacking resilience.
As a mum I would have told her to make the most of the situation and go and enjoy herself, use Ai for great ideas, use Google maps for getting around, have a look on 'get your guide,' for any intersting trips, see if theres any 'free' walking tours ( remember to top!) go to iconic landmarks , take loads of photos, find somewhere great to eat, go and have a fabulous time exploring and come home and tell me about your adventure!!!
Its not about being tough , its about building resilience, encouraging a sense of adventure and turning something negative into a positive expierence.
I agree with some who said about sending a friendly text( if the argument isnt too serious, I guess it depends what it is about) to try to reconnect.