If anything, she should talk to her ob-gyn if she's concerned about the impact of estrogen on her ability to lose bodyfat. But also the assumption that that is the source of an inability to "firm up" is probably wrong. There are a million other places to look to try to "firm up" more - primarily diet & training. If what she's been doing isn't giving her teh results, then perhaps change it a little - different training, cut down on cardio & lift harder, try carb rotation instead of perpetual atkins, etc.
If you are thinking of antiestrogens like guys take - l-dex, arimidex, nolva, etc. - these arent' needed for women because we are supposed to have estrogen. I'm kinda not really sure what it these would do to her menstrual cycle either. Is she on birth control? This is a source of estrogen regulation that will often result in a certain level of bodyfat retention. Many female bb's will NOT use b/c and opt for an IUD or non-hormonal method of birth control in order to achieve competition leanness. For a non-competitor, is something she's interested in doing? (... lifestyle, relationship, etc. issues can be a consideration).
19% bf is not ridiculous & 120 at 5'6" - (I think I was that weight in Jr. High?? and that was the last time I saw that weight...) is fine. If she wants to look "more firm" -- build more muscle instead of trying to lose fat. The result is a tight, healthy, strong look while retaining a healthy amount of body fat.