I've had this IRL discussion too many times, where the non-veg person says that a lack of meat affects their sleep, energy level, digestion, etc. I'm not a dickhead, and I don't argue with people's lived experience, even if it feels very sus.
I've done a good faith search for evidence of how certain people might have negative health effects from not eating meat, and nothing turns up. (Short of just having a bad veg diet) Maybe I'm missing something. It's just frustrating how often people use this as an excuse, and they're often anti-vegan in their wider ideology.
"skill issue"
IMO, people who experience health effects from cutting out flesh either failed at meal planning or they're sensitive little babies who couldn't handle a change. I empathize, western pattern diet depends on flesh and I guess it's jarring to make a big dietary change of any sort (though it wasn't for me), but that is what you're signing up for. Maybe ask about specific things, like "how did you get enough protein" or "did you take B12" and find obvious mistakes. Broader level though, animals still don't want to be killed even to give a carnist a better night's sleep.