As part of a healthy diet, moderate intakes of soya foods (such as soya milk, tofu etc) is considered perfectly acceptable as part of a healthy diet.
However there is no evidence that soy or phyto-estrogen supplements prevent or cure any form of cancer. Some studies have indicated that in supplement form phyto-estrogens may in fact stimulate existing tumour growth and antagonise the effects of some cancer treatment. Therefore soy or phyto-estrogen supplements should not be taken.