Although one can duplicate a DVD rather easily. that's not the problem.
What I recommend, which is 100% legal and also a more flexible solution is to copy the DVD to a USB stick.
You do realize that the installation procedure is going to ask you for a 25 character Product Key.
Let us say you let your friend use your 25 character key.
When he gets ready to activate his Win 7, he will have problems since MS checks their database to see if the key has already been used.
That's the practical side of things. The illegality is another problem.
1 machine = 1 license
1 license = 1 machine.
You paid, not for the DVD, but for that 25 character product key.