This is most likely a parasitoid Chalcid wasp (Eulophidae from a genus related to Melittobia), who lurks until it can access unnoticed to an egg (inside a paper cell). It is quite possible that the standing female was indeed watching the tiny intruder. But was she aware of the danger, or did she simply mistake it for just another small ant? One of the intriguing enigmas some good pictures like yours sometimes point out.