As someone who has/had breast cancer and had to have a double mastectomy, I find the term 'top surgery' deeply offensive and an attempt to make something life-changing more anodyne and normal. But it is not. It's a major, and to a degree, radical lifestyle choice, one that cannot be reserved. I actually think it's an extreme form of body mutation and always a sign that other things are going on.
While I chose to have a double mastectomy, it really was not much of a choice. However, I did choose not to have a reconstruction, nor do I wear any falsies for a myriad of reasons including not being able to keep my nipples and the at the 'new' breasts would have no sensation. Truth be told, I find it rather liberating being flat-chested. To quote others, I am still supremely feminine without breasts. Indeed, breasts do not maketh a woman, but, if at any point in the future your friend wants children and wants to breast fed, they cannot.
A mastectomy is irreversible. It's also not just about not having breast, there will be zero sensitivity across the chest, breast bone, up to the clavicle and size of the rib cage. Is your friend busty? Is so, the scars will be large even if they surgeon is an artist at their craft. Does she want to keep her nipples? That's another complication that can cause issues and potential loss. There's so much that can go wrong that I know I wouldn't choose to do unless I had to. Does your friend note that the better surgeons are onco-plastic surgeon who only work with cancer patients? And, to be frank, and surgeon who is performing a surgery that is not needed on any medical grounds whatsoever is to my mind, not anyone I would want near me, let alone operating on me.
Your friend has choices. Pre-cancer, I rarely showed off my breasts or wore anything low-cut mostly because I was uncomfortable when I did. And even though I seemingly made a choice to have a double mastectomy and feel liberated, I miss my breasts. I think it's really important that your friend learns to love themselves and accepts the body they are in. Being non-binary is about not conforming to gender stereotypes, that's all. This does not mean surgically removing parts of one body.
Is your friend in counselling? If so, they need to explore why they feel the need to do something so extreme, because it is extreme. And chances are once they go through with the surgery, they will realise it doesn't make them happy, nor the person they seem to think they are.