Divorce often leads to a serious disruption in the life of the family pet. I commend your honesty in describing your concerns for Toby’s future. Start by making a list of all of Toby’s needs, such as food, shelter and attention. Then discuss these needs with John as part of your separation agreement.
In the meantime, please avoid negative references to his girlfriend—that will only inflame him. Your distressing thoughts about his new sexual partner are irrelevant to solving Toby’s problem; do not make Toby an excuse for you to express your hostility toward John. The momentary relief this may provide is not worth the harm it does to Toby’s chances for a new home. If John has behaved responsibly toward Toby in the past, he will probably continue to do so. Also, when your children visit their father, Toby’s presence will be an asset.