This is my take on this recent event.
First, I must say that...yes, it is obvious to me that the relationship between Alba and Diana is finished. Summaries of their conversations from forum members who understand Spanish have made that clear.
I'm certain the guest is Diana's "other woman". I think Diana invited her to the apartment for a number of possible reasons. I might be wrong on all or some of them, but I think they all are possibly correct.
1. Diana wanted Alba to meet the girl.
2. Diana wants to have sex with the girl in the apartment.
3. Diana wants the girl to move in if/when Alba moves out.
4. Diana wants Alba to like the girl...possibly to have a threesome.
To her credit, Alba was nice about the whole thing. But, I don't think she was ultimately attracted to the guest. I do think the two of them came to an understanding and are not enemies.