This one seems pretty clear-cut to me. Laws are not relevant to whether you write the discipline referral or not. That's your choice, and you could consult your boss if the other teacher gives you trouble here.
The important thing IF you write it is simply to make sure you make clear in your writing that your information is second hand. If you write "the student did X", you could be legally responsible for defending that claim. If you write "Teacher Y says that the student did [X]" then you are making claims ONLY about what the teacher said. If it were ever to come up as a legal issue, you would not be responsible for knowledge of what the student said, and this would be deferred to the teacher who saw it, and whom you mentioned in your discipline referral.
My personal opinion is that you should just insist the other teacher write his own discipline, for an issue that s/he witnessed him/herself. But, provided you make clear WHO is making the claim of what the student did, you do not create a legal issue for yourself by writing it. You are simply reporting what others' told you, and they are still responsible for the content of their own claims.