How to use an old MacBook Pro as a display?
Unfortunately, Target Display Mode doesn't work on MacBook Pros. However, this thread here describes various ways to use the MacBook Pro as a screen. In a nutshell, it says to use a third party app such as Air Display or Screen Recycler (both of which, it appears, operate over a network. The thread suggests connecting the two computers with a thunderbolt cable and thunderbolt network to speed up the connection. For more details, see the link above.