You are asking a rhetorical question, I assume. The answer is obvious. Men are inclined to be tempted by attractive women and the sight of them, and a variety of them. Even if they are otherwise happily married. Avoiding or resisting tempting circumastances is the proper thing to do, but sometimes we needs God’s help to do that. This cannot be lost on you, can it?
It’s not her fault, but it certainly is her problem. She’s the one who’s unhappy about it. Here’s the real deal - you can remind, monitor and/or push your man to seek help, but do you want to know what will really fix it? Immediately?
Open communication about sex, open communication in general, a willingness to expend sexual energy and creativity regularly, and an interest in your own desires as well.