Rei's constant habit of gossiping about Mahoro behind her back and giving everybody he knows the impression she's a lazy and bad wife is par for the course at this point.
Having found at now that Mahoro wasn't being 100% honest about their marriage, Rio is taking it as justification to go after Rei to get back at Mahoro in a petty way after agonizing and comparing her marriage to Mahoro's this whole time.
The author also making it blatant how in her own head Rio is, when Rei's co-worker outright says, "of course you can never tell what's real or not, this is 'Rei' we are talking about". She's only met Rei like 3-ish times up to this point so she has no idea how much he could be exaggerating and basically self-confirms that he's telling the truth.