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 work?
In general, for other times (when/if it happens), ensure that:
- The work division / assignment is formalized, and it is clear that the co-worker is overstepping to take up your job - you did not ask them to get it done.
- Keep your manager informed that the work which was assigned to you to be done at a particular time, is already worked on by the colleague and you have no part in ensuring the correctness / applicability of the result / outcome.
FinallyAlso, next time the planning meeting comes up, bring this topic up and mention that if they believe they have less on their plate, they can either look for more work to be assigned by the boss / manager, or talk to the boss of utilizing their free time effectively. Stepping into someone else's assignment actually messes up the plan, instead of making it better.
And, I am worried about her as she is supposed to rest over holidays but still working and may be he feels some sort of competitions or something between both us.
I'd say, until proven otherwise, assume good intentions. Maybe by doing your work, they're trying to be nice and making themselves useful. It's true that by doing this sort of overstepping they are actually creating problems instead of helping, but it may also be an honest mistake.
Talk to them, tell them that what they're doing is actually not helping. They may realize the mistake and never do it again!