I had a tiny pleural effusion that grew to large after my surgery. Since it did not resolve on its own, they had to drain it. The pathology report came back negative; so, it was assumed that it was a reaction to my surgery. So far, I've not had a recurrence.
There can be many reasons for someone to have fluid in their pleiral space, infection, viral, irritation of the plueral lininng of the lungs and yes cancer in the lung(s). You should see what the Cat Scan of the chest showed if she had one. If the cat scan was negative for any suspicious masses blood work would be able to give the Dr's an idea if this fluid is an inflamatory response.
Good Luck