Our Chaz chomped on half a fat ball that had been dislodged from the bird feeder last year. Instead of giving him his normal food we gave him some boiled chicken that evening and noticed that he was drinking water more frequently than normal.
The following morning he was very loose and this continued through the day as well as drinking more, but his poo started to become more normal as the evening went on. We substituted boiled chicken for his normal food again and the next day his poo was back to normal as was his water consumption.
We mentioned it to the vet at his next checkup and were told that we'd done the right thing and that a single fat ball shouldn't do any damage but that problems had been known of if dogs had eaten a large number of them or were fed them over a period of time.
Hope Ernie comes through this as easily as Chaz did
Bob.