Translation:This project is very stressful for her.
This is one of those strange differences in English and Polish sentence contruction...
The literal translation seems to be "This project is a big stress for her" which actually works perfectly fine in English, and retains stress as a noun instead of translating it into an adjective
Could you also translate "This project is very stressful [for/to] her" as either Ten projekt jest dla niej bardzo stresowy" or even use dative case and say "Ten projekt jest jej bardzo stresowy"
No, I don't think this adjective works in the Polish sentence, and the Dative construction is a rather unusual one, it may work sometimes, but that probably will sound dated (see? Dative, dated, hehe).
Perhaps "stresogenny" could work, I can see that English has a similar word: "stressogenic". And "stresujący" for sure.