Co-author conflict after paper got accepted
It is unlikely that a journal will publish a paper if one of its authors states that it should not be published at this point (for various reasons). However, it would also be more than questionable for you to demote the other author to only be included in the acknowledgements section (at least if you did not do everything that you could to resolve the situation).

So that leaves you with trying to resolve the situation. One way of dealing with such problems is to elevate the issue. Possible points of contacts could be the dean of your former department or your dean of studies. In some places, where you have a secondary advisor whose role is to help you in case of precisely such problems, then she/he should be your point of contact. 

To prepare any of these meetings, make sure that you have a statement of what happened that it is as accusation-free as possible, while trying to stick to the facts. While such issues are better discussed face-to-face, preparing the statements helps with getting your point across in an organized way.