This is a great question. I’ve written at more length on the topic here.
I’m gonna steal a paragraph or two from my previous post, because it’s a lot more clear than anything I can put together at this hour.
First, a bit of background. According to the New American Oxford Dictionary, information is “what is conveyed or represented by a particular arrangement or sequence of things.” Note that information is communicated by a recognition of some sort of pattern. The burden of conveying information is thus on the recipient to translate a signal into something meaningful, as opposed to the creator of the signal to create some pattern. According to the above definition, there doesn’t even have to be a creator of the information. How can that be? Here’s an easy way to conceptualize it - if you want to know what the weather is like, you can stick your hand out the door or walk outside and feel the temperature. Did all of the clouds gather together, craft a message, and then beam it down to you to receive? Of course not; you simply interpreted otherwise meaningless signals (i.e. the temperature, the arrangement of the clouds) and recognized the pattern (say, that it is sunny and warm). Thus, nobody ‘created’ anything but you still gained valuable information about the world.
There’s an important consequence of our definition of information, and that is that information exists only insofar as there is a way to interpret it. Information depends heavily on context. Hearing beeps may mean nothing to you, but would be very meaningful to a Morse Code translator or a nuclear scientist with a Geiger counter. The same goes for our DNA. Simply, DNA is a very long, highly regular strand of phosphorous, oxygen, carbon, nitrogen, and hydrogen atoms. The molecule itself carries no information that would be obvious to you or I. Our bodies need some way of interpreting the sequence of base pairs and turning that into something useful, like proteins. This is exactly the function of ribosomes in our cells. Ribosomes take raw genetic material and translate it into a recipe for proteins. Neither the nucleic acid nor the ribosome have any agency, consciousness, or intent, but the cell still gathers information from its DNA and puts it to use.
This takes care of the theoretical side to your opponent’s argument. Talking about “destroying” information in the genome is rather absurd. There are numerous types of mutations, and while they may alter drastically the original DNA sequence, no information has been destroyed; it’s just different. The new sequence might not be of much use to the cell, but it’s information nonetheless.
When it comes to the specifics, your opponents are also demonstrably wrong. We have tons of specific examples of mutations (detailed at the genetic level) that are obviously beneficial to the species. See here, here, here, etc.
Mutations, and to a lesser extent their impact on natural selection, are well understood. Anyone who claims otherwise is either ignorant or dishonest.