I was myself in a similar situation ; I was working well with somebody I regarded as a friend , I was in a managerial position and I had some problems with some members of the ‘team’ which needed disciplinary action and I thought my friend was supporting me .However , one she revealed her true colours and it appeared that she was on the side of the people causing me problems ! I don’t think that to this day I have recovered from it , i was completely shocked , hurt , angry etc
I even went to counselling but basically had to grieve over the relationship and yes we carried on working together without being friends but that was hard too because you also need trust in a working relationship.
I have now retired , she has got my job , we haven’t kept in touch needless to say and sometimes it still hurts to think about what she did and also , I just don’t understand it !
So. , you will get through this but it will be painful there is no denying it ; sorry but this is the truth , basically I think that in the world of work we have no real friends.