Here's my attitude on the matter ...
I've long posted that I recommend against increasing bust size artificially. But I'm going to refine that.
If the woman would feel better about herself, she should go for it
Here's the caveat though. A woman should consider wearing bra enhancements to get that added cup size, especially if people aren't seeing her nude regularly. That will do similar. In fact, it's a great "try before you buy" approach as well, at least when clothed. Waredrobe can make a huge difference as well.
Of course, if its her partner asking, well, I guess that holds a lot of weight as well.
In any case, women should feel content with their body. If that means adding bust artificially, then so be it. Personally I don't prefer it, but it does "round out" some hourglass women quite well. And I'm more than content with my wife and the women I go for.