Um ... "tone up a bit and shed like 5 lbs of fat" --- for a lousy 5 lbs, I'd stick with diet. A quick tweak on the diet or a couple extra cardio sessions will get her there.
Has she ever done a cycle before? Obviously steroids are not meant to be diet pills. Primo is a little more unpredictable when it comes to women and responses. If any cycle, anavar is going to be more predictable with fewer sides.
Regarding breast size, just as you can't spot reduce bodyfat, say for buns or hips, you can't make the bodyfat stay in one spot and not the other. If she loses bodyfat it will be lost in a proportional manner across her body. But not that women tend to have more fat cells in the lower extremities than in the upper body, so even if measurements show bodyfat is being reduced, it will be more apparent on the upper half than on the lower half. To that same end, 5 lbs of bodyfat isn't all that much, so she shouldn't see huge amounts of bodyfat loss around her breasts. But since I don't know how lean she is to begin with or how lean she is on her upper body, I can't really tell you how it would turn out.
Back to the primo - how much does your wife know about cycling? Can you tell us her history & intended cycle? Also are you sure that the primo is real? (There's been a LOT of fakes floating around for a while now...) As I said, I wouldn't recommend to anyone looking to "lean out alittle" to go the steroid route. Its not a quick fix and is not intended to be a diet pill.
With regard to the Nolva - we've been having some debates as to the value of using Nolva. The medical literature only shows studies done on post-menopausal breast cancer patients. I would also recommend staying away from nolva as well. I also dont' know if the information you (or she) are drawing your current knowledge about primo & cycling is relative to male experiences - it ain't the same for women, so please make sure you have LOTS of information before she makes any decisions about using steroids for this small body change.
I hope I am not coming across as sounding like "you dont' know anything about steroids, so just don't do it" -- but I am anticipating some of the typical questions for first time female cyclers or guys who are asking for their girls. Steroids can result in things that you (or she) didn't expect such as excessive acne, bloating, etc. So just want to encourage you and her to make an educated decision keeping in mind that its much easier to achieve reduced bodyfat via diet & training than steroids.
Here's a recent thread regarding use of Nolva (aka tamoxifan) by women other than for breast cancer therapy:
http://www.elitefitness.com/forum/showthread.php?threadid=201693&highlight=nolva
Please Scroll Down to See Forums Below 
