Haruto... The more is flirting is Mahoro the more I view him as a jerk.
Since the first time he meets Mahoro he is constantly making joke about them being a couple or him being interested to her while knowing the situation about her husband.
The first time it was to lighten the atmosphere and I was okay with that but now I found it inappropriate and tactless. It feels like he's preparing Mahoro to accept him as his next partner.
Also if he's like that with other women he meets, Mahoro better not be the jealous type.
In another hands, his jokes are a good way the taste the water. If you take his jokes seriously and you're interested, good for him, he just needs to wrap it off and the make the finish move. But if you're not interested or he lost his interest on you, he can play it off as a joke, lol.
The flirt disguise as jokes. Such a player move. We don't even know his love situation. He's not the focus here so yeah...
And if Mahoro is really planning to divorce her husband, having Haruto helping her to catch evidence of her husband cheating can play against.