If there is any remote likelihood that your friend will need medical treatment when she's away then she needs to tell her insurer - if she had no idea she would be diagnosed when she took out the policy, then (IMO) as Karl says she should be OK, but even if she was still going for tests when she booked the holiday it should have been declared. Perhaps she did? However, when our family members have been diagnosed, the cost of travel insurance has rocketed - it all depends on the individual and their prognosis. Is this something your friend would realise, do you think? - just curious as to what you might plan to do, to advise her. Time will be of the essence - perhaps the best step would be to contact the tour operator, if she is part of a block policy which they have organised.