We got a pair of Gouldian finches on Sunday - both male - and they seemed well and happy their first night and morning with us. Yesterday afternoon/evening, one started sitting puffed up with his eyes closed. He would go from this behavior to hopping around and eating like normal, on and off, the rest of the night. I tucked them in and he was still pretty quiet and puffy, so I was worried.
This morning he was worse, and slept for a few hours perched on a short ladder about an inch or two off of the cage floor. A few hours later he fell off the ladder, so I moved him to a more comfortable folded paper towel on the floor of the cage - I knew it was grim - where he laid for about 10 minutes before flopping around to the other side of the cage and dying before my eyes.
I have NO idea what happened! He was either sick when we got him and hiding it (which I know they're good at) or perhaps the stress of his new environs and diet caught up with him. He wasn't even with us 36 hours.

I've been watching his buddy all day and while he was obviously stressed when the other bird was dying, he's been sleek and fairly active for the last 5 hours. He's eating treat seeds and millet, and responds when I talk and whistle to him. Even chirped a bit ago - he might be okay.
Does anyone have any advice on what to do for him? He seems well, but so did the other guy until very suddenly. I know they don't like to be alone, so our plan is to wait a couple of weeks to ensure this bird is healthy before seeking another friend for him. I'm getting the number of the man we bought them from tonight and can call him as well, but I'm not sure I'll get a good answer.
We're not new to birds, but these are our first finches. I've been scouring the web for hours (including this site) and am armed with some information - would love to hear what the folks here think.
Thank you so much in advance,
Malyssa