As a radiologist I work with high powered magnetic fields and strong radio frequency waves (just like microwaves) as part of MR imaging on a daily basis. Our radio wave power emitter is a couple hundred times the power of that of a handheld cell phone, and we put patients whole bodies or body parts into the magnets all day long for sometimes up to a couple hours per patient. The concern with radio waves is that they are indeed energy and can theoretically deposit that energy in your body, generally as simple heat, and the closer your body part is to the emitter of the radio waves the more likely there is to be energy deposition of a greater degree. Male gonads prefer cool to hot environments for optimum function, so theoretically your friend is correct. We never have discovered any adverse effects to our high powered magnetic fields and high powered radio wave emitters now with nearly twenty years experience with the technology. Theoretically, there is a risk but practically it is negligible. Because the power of the cell phone is so low by comparison I would not fear "frying" your nads with your cell phone in your sporran, any more that I would fear frying your eyes or brain with your cell phone when you are talking into it while holding it by your ear.