For the record, personally I think Faye just doesnt like it that Angus isnt there.
Not that she's in any way upset that they said the words to each other.
When I read yesterday's comic, my first thought was also that she realised how much she would miss him. It's not insignificant that she realises how attached she is at the very moment he leaves her. Leaving for the weekend, yes, but nonetheless leaving her. She's now left with this sudden awareness that she loves him, combined with her trust/abandonment issues, and she is all alone. That is obviously going to trigger emotions that she would struggle to cope with. The point of a coping mechanism, however, is to let you cope. At the moment she is drunk off her face, much less inhibited, and is staving off her issues for the time being. So of course, it's not going to come through in her behaviour that there's anything wrong. She was drinking so that even she would forget anything was wrong.
And Marten is perfectly justified. He may be her flatmate and her close friend, but he's not her mother, and he's not her partner. Marten had plans for that evening, plans involving other people, and plans which from the sound of it also were to involve Faye. If she comes in drunk and rambunctious, babbling in faux-English-gentspeak, that's no indication that he is somehow obliged to cancel his plans, only that she is clearly in no fit state to go out.