My husband sounds the same as yours. He is a very nice guy and oblivious to when someone is flirting with him. Im a very jealous person so I might not be the right person with the right answer. But if it were me I would be like a dog and marking my territory, making sure she sees it. Being all lovey dovey and practially attached at the hip anytime she is around, while sending out " back off bitch he is mine, if you touch ill have no choice but to kick your ass vibes" I like to be subtile at first when it comes to the feeling that someone may be invading things that are mine. But I never let my guard down when there are vultures circleing. I sleep with one eye open.