Hey @KHaskin, thanks for sharing! This is a huge bummer, but you actually aren't currently able to reply to emails from the HubSpot mobile app.
However, you're not the only person asking for this functionality! Here's the HubSpot Ideas request to add reply and forwarding capabilities — it looks like it is currently in Planning, so it's at least on HubSpot's radar! I encourage you to upvote the post and comment your use case. If HubSpot sees that more people are interested in this feature, they'll hopefully make it a reality!