I'm afraid you would be lying to them. You could tell them what you want, but the proof of ghosts being real does not exist. There are plenty of fake pictures, fake videos (documentaries included), people who are paid to go in TV to ramble on about what they think they saw, and above all fake psychics who claim to channel the messages from the dead. Not one person, place, or thing has ever turned up any concrete proof that ghosts are real. Try to search for a person named James Randi on the web and read about him. Perhaps it would be best to not call your friends names, but instead state your belief/ opinions to them to start a conversation and see if you can't find something else you all agree on.