Cross posting of questions between sites is discouraged since it creates noise and spreads answers across the different sites. If the question were on topic on PMSE, you could leave a comment asking the asker to delete the other question. You could also flag it on the other site for removal.
I went ahead and flagged it on Stack Overflow for removal. Thanks for pointing this out.
I feel like this question is -- at the very least -- off topic for PMSE as it doesn't appear to be about project management. Also, the "design A" and "design B" abstractions make it hard to definitively answer this question since we have no context to decide which coworker the asker describes is right or wrong, and I don't see it helping future visitors. Based on this lack of detail and the nature of the topic, we could close this question as not constructive, not a real question, or off-topic.
Since our site scope is in a state of change, I must ask, what do you think about this question?