I presume she is at risk for developing bowel malignancy at some time in the future. So, unless she is screened very regularly, then if she is accepted as a blood donor, she might be donating blood while she is in a cancerous state. Now, is there evidence of the ability to transfer cancer from one person to another via blood transfusion? This has been looked at historically and
Despite the many millions of units transfused
since the advent of allogeneic blood transfusion,
there is no evidence to support the theoretical
concern that cancer could be transmitted via blood.
Apart from the United States and Australia, though,
most blood services still practice a precautionary
exclusion of donors with a history of cancer based
on experience in organ transplantation and, to a
lesser extent, HSCT
Can Blood Tranfusion Transmit Cancer? A Literature Review
So, the UK blood transfusion service is adopting the same precautionary approach.
But there is evidence that as yet unknown carcinogenic agents are transmitted by blood transfusion.
In a large cohort of UK women, transfusions in the 21st century were associated with long-term increased risks of liver cancer and non-Hodgkin lymphoma. Some of these malignancies may have been caused by carcinogenic agents that are not currently screened for in transfused blood.
Cancer risk among 21st century blood transfusion recipients
A large study from Denmark and Sweden reported relative risks 5–9 years after blood transfusion in 1992–2002 of 1.86 for liver cancer and 1.20 for non-Hodgkin lymphoma
Cancer incidence in blood transfusion recipients.
The likely candidates are Epstein-Barr virus and hepatitis G both of which are not screened for in blood donors.