It's a preposition.
There's actually a few different prepositions that would work there, with slightly different shades of meaning. I'll provide some examples with a shorter sentence.
She was upset at her husband
^ Husband did something to upset her, and maybe she is angry at him. For example he said something rude in front of dinner guests.
She was upset over her husband
^ Something was happening to her husband, or her husband was involved in some situation which upset her. For example, he is sick.
She was upset because of her husband
Husband did something that directly upset her. Perhaps he was pushing her buttons that morning.
She was upset with her husband
^ She is definitely angry at him. Maybe he forgot her anniversary.
She was upset about her husband
^ She is definitely not angry at him. Something happened to her husband that is upsetting. Maybe he is being targeted in an upsetting lawsuit.