I really don't know how to react to this! As I feel upset as well as have little bit worried about co-worker as well!
Don't overthink it, be happy that the work is done. Enjoy the rest of the holidays.
Could you please help me how to tackle this co-worker as this is not the first time, he has done this: working over holidays / weekends and take up my ...
It doesn't matter why they ended up working when they weren't supposed to. It isn't any of your business. Their plans could have fallen through, a vacation got cancelled, family changed plans, who knows? It doesn't matter. If its square with their manager it is a done deal.
I was supposed to finish month-end activities on 27th December, upon
my return, ...
There are a few different things that might concern you about this. If you take a look at what you're thinking and figure out which of these are problems for you, you'll be able to make a more targeted solution.
You're worried about your co-worker's well being.
You're worried it'll reflect badly on you that someone else did your work.
A simple oversight.
That person is depressed or has some other issue.
She may not like the quality of your work.
Fourth possibility. That person is committing fraud. In auditing, one clue, that someone is committing fraud, is that they're not taking their vacation when they're supposed to, and they're not letting others do their work, ...