Cut hair badly and the whole composite looks fake. The trick is to preserve partial transparency rather than forcing every pixel to be either subject or background.
Start with Refine Hair
The hair refinement control specifically protects thin, semi-transparent detail instead of smoothing it away.
Kill the halo, gently
Halo reduction pulls the background colour out of the fringe. Too much of it eats the strands, so raise it until the halo disappears and stop.
Brush at low opacity
Around hair, several light strokes beat one heavy stroke. Drop the brush opacity to around 30% and build up.
Check against a dark background
Fringing that is invisible on white jumps out on black. Preview against both.
If the AI removed too much, see the restore guide