I'm old school so I don't use emojis. I think I'm so old school that when I see those, I think of emoticons, instead of emojis. But to get heated/stressed about something like that is laughable. If it's just a direct reply or a conversation between two people, that is perfectly ok. I would hope people avoid using it when replying to all, including their bosses. I have seen some even include them in their email signature, so it's always there. Your friend needs to get with the times, and learn to bend some norms.