I was given a fish tank and 4 fish (quite largish goldfish). I didn't want it, but it's a favour for someone. So i'm a complete fish novice.

This morning one of the fish is lying on it's side on the bottom of the tank. The other 3 fish keep nudging him or just floating very near him. He's currently on his side hiding behind a 'log'. It's awful. Clearly all is not well with him. He doesn't appear to have any visible issues.
The water is slightly alkaline, but no more so than when they first got here a couple of weeks ago. I have added a ph down treatment.

He's nearly 8 years old according to his previous owner. This seems oldish. I can't ask the previous owner for advice - honestly, i've had them less than a month, and one is already looking shaky.

I don't mind too much if he is just dying because he's old. However tonight we are leaving for a wedding and not returning home for 12 days. What if he dies? He can't just float around in a tank for that long. The other fish will get traumatised. It's too yukky to imagine. We don't really know the neighbours well enough to ask them to check on a potentially dead fish.

HELP. I really don't have a clue what to do.
Thanks in advance.