This is a great point that actually makes me lean toward the latter option. Imagine being the boyfriend whose with her during the process, having to suppress your true feelings about the reduction because you want to be outwardly supportive of her decision.
She breaks the news, you give her a hug and say "I support whatever you want to do" but really you're holding back tears and praying to god that she will somehow change her mind.
Your friends who know how stacked she is all give you that sympathetic "sorry for your loss" look if they find out what's happening.
The last time you fuck before the surgery, you have to maximize your enjoyment of her tits without making it too obvious to her, then mentally say goodbye to them forever.
Then after the surgery she shows you her smaller tits and asks you how they look, and you have to say "great!" to make her feel better even though they're a shadow of their former glory. Every subsequent time you have sex and go for a squeeze, you have to hide the look of disappointment on your face at how much less her boobs fill your hands now. As she rides you, you have to access your spank bank for memories of her old tits bouncing on top of you just to feel the same excitement as you used to.
I don't think I'd be able to lie to a woman like this without it eating away at me. Having to be outwardly supportive of a decision that brings me so much sadness sounds like hell.