"This is her day."
Translation:यह उसका दिन है।
No. "Uska" is correct. Here "uska/uski" has to agree with "din" and not the person being referred to. So regardless of whether we're talking about a man or woman, the sentence will be "ye uska din hai".
My native speaker boyfriend says यह दिन उसका है is also correct. Is he right or using bad grammar?