I own a copy of the original release of Dark Souls 2 on Steam, but my friend was wondering of getting the Scholar of the First Sin edition on Steam. If I want to be able to summon him in my game, would it be possible considering we have different versions?
If your friend is buying the DX 11 version, you will not be able to summon him, because the DX 11 Version has a different server.
If he is upgrading from Dark Souls 2 (DX 9) to Scholars of the first sin (DX 9) you will be able to summon him because you are on the same server.
Here's a picture that has a full comparison of the different versions:
"Dark Souls 2" and "Dark Souls 2: Scholar of the First Sin" edition are not compatible with each other, the DX11 version has some items and enemies that the DX9 version does not. As such, co-op does not work between the two versions.
If you're still in doubt about the versions, here are links to the two different versions on the steam store. Dark Souls II and Dark Souls II SotFS. There is a 40% discount for people that own the DX9 version.