I am a commercial pilot (that is the name of my certificate, by no means am I an airline pilot), so I fly small aircraft fairly often. Whenever I fly, I make sure that my phone stays turned off because depending on the plane and the cell phone company, whenever I have an incoming call or text, it creates static over the radios. I've never known it to mess up my instruments at all, but I'd imagine that if I had a plane full of people who were all on their cell phones at once, it might screw something up.

In this case I think that the flight attendant should have been more considerate and actually explained why it couldn't just be in "airplane mode," but on the same hand, the customer should have turned off his phone when asked to.