It depends on how continent the colostomy is. If she always needs the bag and may otherwise leak into the water, she should use the mikveh with the bag in place. If she has some control over the flow and can assure that it will not leak during the few minutes in the mikveh, then she should immerse without the bag. The infection control issues should be discussed with the mikveh attendant in advance. Depending on the system, it might be best for her to be the last one to use the mikveh and for the water to be changed before the next day. If there is a filtration system, it is probably still best for her to be last to immerse, but it may not be necessary to change the water.