It's stuff like this that makes me not open unhatched eggs.
You either incubate it until it hatches or dispose of it humanely. There's a reason why the broody abandoned it and you may nurse it and find it does great or it could pass soon after hatching. Decide how much effort you want to go through for this one potential chick and go from there.
If you have a bator you can pop it in there and see if it will hatch. Figure it's on day 18 humidity levels. Once it hatches it will be behind in growth compared to the others so you will need to make a brooder with heat lamp and raise it until it can be merged with the others at maybe a week old? The broody may reject it then too so be prepared to brood it to six weeks.
Or you tuck it into the nest after the broody goes to bed tonight and allow what happens to happen.