Yes, it would be advisable to keep your mutuelle up to date.
Although a cancer diagnosis is classed as an affection de longue durée and therefore means that your partner is entitled to 100% cover, this is for his cancer treatment. Any other illness that he has will continue to be reimbursed at the usual rate, eg. a broken leg. In addition, various charges are not covered, for example, the daily charge for a hospital stay, medicines which are not on the approved "list", consultations with an oncologist or other doctor who charge more than the standard rate. This is all described here if you speak French:http://www.ameli.fr/assures/droits-et-d ... bourse.php
I am sorry to learn of your partner's diagnosis, do please use the Forum or other CSF resources for support. We are here for you. And do get back if you have any other queries on this.