If you're trying to replace a preinstalled built in optical drive you need to know the make/model of the laptop and use a drive specifically made for it. It's not like a standard one size fits all as with desktops. The unit needs to be sized properly, the connections in the right place - it's a proprietary design. That is if there's even a bluray option for that laptop, depending how old it is and what standard equipment or optional upgrades were offered there may not even be a compatible bluray drive for it.
You could always opt for an external bluray drive that connects via usb, that would increase the chances of it working with your cousin's laptop.