Attracted to my girlfriend’s mom

I was staying the weekend with my girlfriend and her parents. Her father is older, while her mother younger and remains in great shape, a result of her dedication to fitness. My girlfriend frequently refers to her mother as a "pick me" — she's always hated this about her mother.

Despite this, I find myself attracted to my girlfriend's mother. Her more mature features-her fuller figure, her larger, perkier breasts, and her curvier shape-contrast with his girlfriend's youthful appearance. But what stands out most to me are her pretty feet. Despite being a size larger than his girlfriend's, her feet, always pale and pink from a day of work, captivate me. I can't help but stare when she walks by in her sandals or barefoot, even though he knows he's being noticed. To my surprise, it doesn't seem to bother her.

In fact, she seems to play into it. When my girlfriend isn't around, her mom often wears flip flops and wears athletic clothing, tight tops that always reveal more than he expects-sometimes even showing her bra or the outline of her nipples.

I wonders if this is a subtle invitation for attention, maybe because her relationship with her husband has grown distant, though he's never been sure. The tension feels almost unbearable, and I’m unsure of how to handle it. Should I confront the situation or simply continue to offer her the validation he assumes she craves?

Does she enjoy the attention because of a lack of intimacy elsewhere in her life, or is it just harmless flirting? I sometimes wonder if she and his girlfriend have talked about it, as they share an unusually open relationship, but that thought only complicates things further.

I can't deny the attraction, yet I feels a growing guilt, unsure of whether he's crossing a line. At the same time, the attention feels gratifying.

There's a conflict between doing what feels natural and respecting the boundaries that should be in place.