''How would l say thats her thing in Spanish?
eso es de lo ella.
2 Answers
I¨m not sure of your context, so I guess you want to say: Es asunto suyo (It´s her business and no one else´s).
Or something that only she does: Es algo (muy) de ella.
I agree with Julian above; it's very context based.
If you were pointing to a physical object and saying "That thing is her's", I would say "Esa es suya" or "Esa es de ella".
In other contexts (like "that's what she does" or "that's her business and not yours") then Julian's translations are what you want to use.