Hi timily and welcome to the forum. It sounds like a case of poor water quality and no conditioner getting to the little guy. I assume you did a water change when you got him back from your coworker. I realize you'll be busy at work but if you can, it would be best to do a full or close to full (90% is okay) water change every day for a week to help him recover. Since he's weak right now, his immune system is very compromised and clean water will really help to prevent him from getting something much harder to treat, like a bacterial infection. And his age, 2 yrs, could also affect his ability to fight off infection so all the more reason to keep his water clean. He's done well so far, I hope he can continue on to live another year.
The other posters are right. If you can, upgrade to at least a 2 gallon tank. One tank that would look good on your desk is the Fluval Spec, it's very compact. You can fit a heater in it so he stays warm; cold water will, again, compromise his immune system. If you want to stay with the 1g, then it would be best to do several water changes a week. Every other day is ideal but I understand that is a bit of work while you're at work. If you can, try and do 1 50% and 1 100% a week. I think you'll see a big change in his behavior when his water is changed that often.
I really hope the little fellow pulls through and gets better. Please keep us posted.