Same thing happened to me this year, but luckily as spring was approaching. I found a used L16 for scrap price and it lived about three months before it died, then I lost another one of my original bank. I halved the bank and lived off half capacity for another few months before replacing the whole bank at the end of the summer.
Every bit of advice I got told me that a new battery would not play nicely with old batteries, especially in a series-parallel string. The internal resistance of a new battery is much different than the old ones, and the whole bank gets out of balance. However, I never tried it to see what would happen.
With winter approaching, it is a bad time to be living off half capacity, but if your neighbor wasn't daily cycling his old bank below 25%, he could conceivably live off half capacity ant 50% DOD and have a few spare batteries to substitute in if another one dies. My experience was that I had a cascading string of failures after my first one and I'm glad I didn't buy one new battery to try and extend the life of the bank, because it would have been wasted money.