Separate them ASAP.
As has been mentioned, the newt is most probably not going into the water because of stress. The animal seems like it was very badly stressed, perhaps ill, in the petshop, and now being with the Bombina, and at an incorrect temperature is not helping.
The newt needs cold temperatures...this is an absolute must, or else its inmune system, which is now probably almost gone, won´t be able to recover and the animal will fall ill.

Take a look at this link, which contains most information you need:

Caudata Culture Species Entry - Cynops orientalis - Chinese firebelly

If you want your newt to go back to water and live a long comfortable life, you need to provide excelent housing right now so that he recovers. For that you need a monospecific tank, adequate cool temps, enough volume of water, and that the water is in perfect condition. If you do so, the newt will probably do great, they are a fantastic, and incredibly easy species when cared for properly.