See, the problem is this:
If you have big boobs, they are heavy and therefore start to “sag” pretty quickly. Not sag in that horrible grandmotherly way; they just sit a little lower because they weigh more. (How much more? Ask my poor shoulders.)
Since they weigh more, there is less support (hence the un-grandmotherly sag), which causes them to flail helplessly to the sides a bit. When you lay down, for instance, most of the boobage is by the underarm.
With unsatisfactory support (and a lot of nice brand-name bras just don’t give the right support), they are sitting slightly higher, but they still hang to the sides.
Hence, big boobs yet no cleavage.
The best way to remedy this situation is to try on as many bras as possible when bra-shopping. I have gotten lucky and found two styles that give me great support AND rockin’ cleavage (however, be forewarned that in the quest for cleavage, one most certainly sacrifices comfort). Also, tell her to look for demi bras. I have one that I wear ONLY when I have on my most low-cut shirt. My ta-tas have superhuman powers in that bra. However, because of a demi bra’s cut, it generally does not give larger-boobed women a smooth line under sleeker clothes, which results in the eyesore known as “overflow.”
Hope that helps!