Yes, this certainly is a concern. There is a great resource book called Dr. Susan Love's Breast Book. In it, she mentions this very issue. What I have read is that it is not a good idea. After a diagnosis of breast cancer, usually hormones are not advised. This means birth control pills, estrogen for menopause, etc. In order to harvest eggs (not sure if they can do just eggs), there is a significant amount of hormonal treatment one must endure. It might be hard to find a doctor willing to administer those medications to a BC survivor. In addition, she would have to have injections and hormonal treatments if they were to plant an embryo in her at a later date.
Please help her know that there are other options too. First of all, her fertility may not be impacted. I did chemo at age 31 and 5 years later, have not skipped a period. The BC diagnosis is very new to her right now, and she really needs time to get a plan of attack. There will be time to talk about children, whether she can have them naturally, or via adoption, surrogacy, foster parenting, etc.