What if you said it just like that: "I know you love her and don't mean to hurt her, but..." and then maybe share her reaction or what she said? Maybe without criticizing or giving him a suggestion-- just sharing the observation in a non-aggressive way. And if he gets defensive, reiterate, "I know you love her and don't mean to make her feel bad... we both have her best interest at heart... but I thought it was important to share how she responded with you."
And then leave it up to him to ask follow up questions or approach her. You can't control how he feels or what he does about it, but I think it's ok to let him know.
