In my experience people like this think in terms of tools they can use. They read an article about some shiny new thing and watch a conference talk about it and think "wow thats cool, what can I use it for?". Often times their shiny new tech will actually solve a problem, but it might be far from the best solution and it might bring a whole host of other problems.

IMO the key to reason about things like this is to ask "What problem are we solving?", clearly identify it, then brainstorm a list of potential solutions (including whatever shiny new toys this dev wants to use), then run through each solution and list the pros and cons.

Also, try not to start out hostile to the shiny new tech, let the proponent of it do all the talking, just ask questions but don't argue against it, save that for the very last minute. Otherwise it could go a bit sour if you are both clearly on opposing "teams". Finally, try asking the question "what are the downsides of this new technology?".

There was an excellent article published here in the last 6 months about developing in the "problem space" it was about exactly this kind of situation. I just wish I could find a link for it but I can't. Maybe somebody else remembers the title of this article?