I have a friend who is asking me if he buys a 512 GB Samsung 840 Pro, would it be compatible with his Mac Book Air 2010-A1370?
I know nothing about Mac so I want to help him before he goes ahead and orders this SSD from Amazon
No. That is a standard SSD. The MacBook Air has a special type of SSD.
They are available here:
The A1370 is actually a 2011 MacBook Air. If he wanted to replace the internal SSD, AFAIK the only source is here:
Very Expensive. I just replaced the SSD in my 2012 MBA with a 480 GB SSD from OWC.
The Samsung would work fine in an enclosure as an external drive.