Ethics question: Replying to bad reviews of competitors?

Honestly, I think a lot of people who complain about the ethics of this sort of thing are the sort of people who complain about everything. Someone has said they had a problem suggesting to the person there is a solution somewhere else seems like an upstanding thing to do. The fact you benefit from it too makes it a win-win scenario.

Negative feedback or reviews could be an opportunity - first to learn how to improve your product and second to show how much authenticity and resilience your team has. I know it’s not a likely thing to see but it could be beneficial at some point.

It's one of the best ways of marketing. I don't see anything unethical in it.

I would refrain from saying negative about the competitor, but yes, I would not shy away from commenting and mentioning that my product can plug that hole to address the pain point of the client.

After all, this is on centralized store, and everyone has the right to comment :)

I don't really have a problem with it when it's on a centralized location.

so... I think friend did that and got caught by app store and got a warning.

I ignore the review of the competitor. Sometimes, competitors are better ignored, better sidelined, better forgotten.