Depends on the individual tenacity I guess. They don't encourage it, disapprove it , but if you choose to do so and have a GOOD preparation, there is NOTHING they can do apart of nagging the JW witness to drop the friendship.
just a quick note
"Your friend won't be able to get married in the kingdom hall if he has any "worldly" people in the wedding"
This is not completely true as my 2 sisters were married in the K.H. where the assemblies were made at that time , and I was the best man , was present , visible and was greeted by all. OFC they said to my sisters to try and go around me , but when people want the same thing , WHAT CAN DO A IGNORANT COPORATION AGAINST THEM?