You and developer are on the same team. (S)he thinks of a "better" design approach and insists on implementation.
Do you think developer's version of UI will drastically change the final outcome of the project?
If no -> this is a conflict of two personalities. Resolve accordingly.
If yes -> state your disagreement with management and insist on A/B testing of both approaches (yours and developer's). Let the development proceed, measure and analyze - time will be the judge. Repeat this approach three times. If every time you tested it your solution wins and management is unwilling or unable to change the situation it is time to leave the company - there's no future for you there.
Do you think developer's version of UI will drastically change the final outcome of the project?
If no -> this is a conflict of two personalities. Resolve accordingly.
If yes -> state your disagreement with management and insist on A/B testing of both approaches (yours and developer's). Let the development proceed, measure and analyze - time will be the judge. Repeat this approach three times. If every time you tested it your solution wins and management is unwilling or unable to change the situation it is time to leave the company - there's no future for you there.
