I went to Oxford and had some similar issues in my first year, I think partly because I had a family wedding in Freshers week which made my start a bit non ideal.
Changing college just Isn't a thing so she should stop even thinking about that
I never did get the same college core circle that most people had but I did get a good circle of friends in the end. What I would recommend is -
Pick off a few individual people in college - e.g. if she likes her tutorial partner, that's a good place to start because there is a certain intimacy about the tutorial experience. My first tutorial partner is still a friend now - we were never part of the same college social circle but we got on well.
Oxford is full of socially awkward people which in some ways is helpful - my best friend and I met when she came up to me and said "you look as awkward as I feel - want to be awkward together?" There are fewer "cool" people and more people who find social interactions hard and so it's more acceptable to be open and honest about that
There are a lot of formal dinners and events with more structured socialising - do those. You sort of have to talk to the people either side of you at those events
Sign up for a lot more activities - there's a lot more than music and sport. The more obscure the more likely these are to be other students who make it their main thing
Think about student politics or the Oxford Union.
Work the connections from school
What I ended up with was - a social circle from an obscure society which I persuaded a school friend also at Oxford to join with me, I added a lot of friends through my school friend and made a scattering of individual friends in college
I would also recommend inviting people round - e.g. say to her school friends that she is struggling to make friends in college, invite them over for drinks before formal hall in her college or guest night dinner, ask them each to bring someone new
And yes as a PP said, dating is a good idea