I've never heard that, but if anything, what those women are doing is lowering their risk of breast cancer in the right breast, not exactly raising it in their left breast. What the LC probably meant was the they had a higher rate of breast cancer in left breasts than right, not higher rate of breast cancer in the left breast than would be there in the first place. Does that make sense? Switching to formula would just mean that your friend will have a higher risk of breast cancer in both breasts instead of a reduced risk in the right breast if she were to continue to nurse on that side. If that's even true, which I'm not sure of. I was under the impression that it was the hormones involved in lactation that reduced the cancer risk, not the physical act of breastfeeding, but I could be wrong.
ETA: Posted at the same time as the PP - would pumping from the other side then confer breast cancer protection if there was some physical issue that make it difficult to actually nurse the baby from that side I wonder?