I'm using Blender 2.8 and my friend works in 2.79. Whenever I append or link his files there is something like a locked logo front of those objects and I can't manipulate these. What could be the issue?
The symbols resembling a chain link in front of an Object stand for linked objects. They definitely only appear if you link to an object instead of appending it.
If you append an object, you create a local copy of an object from another .blend file, which can be manipulated like any other object. If you link to an object in another .blend file, you can't manipulate this linked object, instead you have to manipulate it in the original file.
To create a local copy of a linked object, click on the object with the chain link symbol, click on "Object" in the "Properties" panel, then hit the chain link symbol on the right side of the Object selector dropdown.
This is an issue with recent builds of 2.8. If you append files from 2.7 layers/collections come in linked. If your able to open the original 2.7 file in 2.8 (make a backup first) save it, then append it into the new project and it will be localised.