Is her partner applying for a visa? I had to apply for a fiancé visa (which doesn’t technically exist in the same way anymore) to come to the UK to marry Dh. I had to supply evidence of our relationship, plane tickets showing our travel to see each other, birthday and Christmas cards we sent to each other, emails and messages, photos, all showing several years of travel and communication and evidence that we did normal couple things like go on holidays, meet each other’s family, talk about long term plans, etc. all documented and organised by date.
We did all this before I was granted a visa to come to the UK after which I travelled to the UK and then we gave notice. If they haven’t already submitted this sort of stuff, then I imagine it will be wanting evidence of this nature. Basically, they want documentation of a long term relationship and all the usual relationship things, travel, time spent together, meeting family, ongoing communication about mundane things. They may also want to interview one or both of them.
Best advice I could give, if she’s unsure what she needs to do and the guidance is not clear, is to hire an immigration advisor through a solicitor who handles immigration cases. I think it was maybe £250 and I used her when I did my indefinite leave to remain application, but she was invaluable in advising on the documentation they needed, how to organise it (it was literally 2 massive ring binders probably 10cm thick each - an f-ing nightmare frankly!), and how to make my case drawing on current immigration law.
I am an intelligent educated person, but I would have been lost without her. Very worth the cost, because if you screw up something teeny the Home Office will use it against you and make life very difficult. So it’s best to get it right the first time.