While the exact cause of breast cancer is unknown, pink grapefruit would be a huge stretch to say it could be a culprit. There are certain risk factors that are linked to the disease (e.g. gender, genetic risk factors, family history, and lifestyle choices). The connection is that estrogen (the hormone that promotes most breast cancers) interacts with grapefruit and early studies found higher levels of estrogen in women who eat a lot of grapefruit or drink a lot of the juice. But of course this is simply an observation and a study performed in 2009 (EPIC study) found no association between grapefruit intake and risk of breast cancer.
Due to the conflicting reports in research it is far too early to conclude that grapefruit or grapefruit juice can cause breast cancer. More research is warranted in this area. That said grapefruit juice has demonstrated multiple interactions with drugs leading to loss of the therapeutic effects or increased side effects. Talk to your doctor if you are on any medications that could be adversely affected by grapefruit intake.