Is he not willing to work together because he doesn't think she is ready, or because he is angry at you? What is he saying to you when you raise the subject? She is young, so if it's the former, I would wait. If he is angry at you, I would still wait.
To assess readiness I would look for signs, like does she let you know when she needs a diaper change, can she pull her pants off and on, is she willing to sit on a potty, like before a bath when she has no diaper on anyway, and can she follow simple directions?
If you are sure she is ready, then I would approach her dad as if he is a reasonable human being, even if he is not. Let him know you are seeing signs of readiness, ask him if he is seeing the same signs? Ask him if he thinks she is ready? Ask him when he thinks she will be ready? If you can't get any cooperation, then you will be on your own.
If this is the only co-parenting issue, great. If not, you might want to seek counseling to develop strategies for how to deal with him. In that case, the potty learning is the least of your worries. When she is older and really, really ready to learn, you can help her learn without him, just explain to her that this is something cool she can learn and show her dad.