New Laptop display showing completely different when plugged into external monitor

[BellyRolls]

Hi, I edited on my laptop and sent via Dropbox some photos to my sister and they are really dark and the editing is totally off on her computer.

I plugged in an external monitor and they are showing like hers did on my monitor, but on the laptop screen look great!

I've calibrated the colours and changed the brightness/contrast but it's not fixed it... any ideas or help would be great!