Just to say I too had a triple bypass. The fluid build up caused me to have breathing problems. The thing is though, your relative has only just had the surgery. As slowgoer says, open heart surgery is so very invasive. Also, the rib cage needs time to knit back together, the scar down the front of the chest is so very tender, there are also holes in your abdomen where different tubes were that also have to heal.
I used a nebulizer for a short time whilst in hospital. As soon as I got home, walking short distances and then increasing the time walking helped my breathing. Also chest expansion exercises, very gentle ones mind you, helped the site to get more supple.
I used a rolled up towel pressed to my chest when I needed to cough or sneeze or anything that I had to hold my breath for. It was called "the baby" by the medical staff.
It all takes time. It's difficult I know, but let your relative know that in time it will honestly get better for her.