Actually, the question is invalid. The asker doesn't believe in evolution. Otherwise, you could just point to any species on the planet and say, "There...there's the beneficial mutations." You could hold up your hands, wiggle your thumbs..."There...there's the beneficial mutations." But since the asker doesn't believe in evolution, the asker is looking for mutations WITHIN species, in which case mutations won't be as easily classified as "mutations". The asker is wondering about two-headed children, six toes, 3-chambered hearts.
The argument isn't worth having. Just answer with, "If you can't see with your own eyes the process of evolution all around you, then there is no example I can give. Mutation is in every living thing on the face of the planet, and even in the rocks, wind and waves. Everything you see is a product of mutation."