I imagine that Anna kept in regular touch with Daniel. I'm a bit suspect of at least parts of the PI's story.
Yes I am sure Anna did keep in regular touch with Daniel. However, given her drugged up state - I wonder if she knew he was losing weight and why. Who knows? That is not to say Anna didn't love Daniel very much. I know she did. His death was simply too devastating for her to get over....but I think she was making progress except for the drugged up state she stayed in. The issue is that Anna was pretty Anna centric....and when one is an addict - it sort of has to be that way.