Yes—this is a common technique. Mount the camera on a tripod and use a slow shutter speed long enough to blur the waterfall, often around 1/2 to 1 second depending on the water flow. If it’s bright, an ND filter can help you reach those slower speeds.

To keep the person sharp, you have two main options:

1. Use flash on the person. Flash duration is very short, so it can freeze your subject while the ambient light records the flowing water. A smaller aperture can also help give enough depth of field for both subject and waterfall.

2. Ask the person to hold a pose very still during the exposure. This can work, but any movement will cause blur, so comfort matters.

Another effective method is to shoot multiple frames on a tripod: one for the waterfall/background and one or more for the person, exposed as needed, then blend them later in Photoshop. This also lets you fine-tune the waterfall shot without making your subject wait.