Is the Companion's saying, "O Allah, I seek refuge in You from all evil and evil-doers," considered mockery if said jokingly after saying "Masha Allah"? And is a person held accountable for a thought saying, "I will abandon religion, it has no benefit"?

Your friend's words are mockery and denigration, and he must repent and adhere to the saying of the Prophet, peace and blessings be upon him: "Whoever believes in Allah and the Last Day should speak good or remain silent." As for your words and imaginations, they are mere whispers (waswas) that have no consequence, so disregard them.