My friend asked me to take some photos of her wedding. Is it okay to hold Sony SEL 135F 18GM lens in your hand?

Bad shoot, not even friends. As soon as you have no experience in wedding photography, look at your equipment. I even conclude that your photography skills are average. Weddings involve a variety of complex still lives, people, close-ups, environment, architecture, documentaries and lighting shooting. You've never filmed it before. I'm afraid you will be blindfolded.