Science is how we answer questions about phenomena in our complicated world. A scientific theory is a hypothesis that has been tested time and again, and has never been falsified. A scientific theory may be upheld as a fact until it is falsified. A great deal of theories exist in science, such as the theory of evolution by means of natural selection (yet to have been falsified).
Well, if we are going to get technical, how do we know anything? I get where your "buddy" is coming from. Did you know, color isn't a thing? How do we know that apple is red, or the sky is blue? It all has to do with how far you are willing to "assume."
Um... Exactly where did we get our computers? Without science, we wouldn't have out smart phones. Without science, we wouldn't have our TV's. Without science, we wouldn't really have anything that's technical. Much is proven in science.
Your Buddy is correct in my opinion. Science is all theory. Some theories have more data to back it up than others. The molecular density of Pluto may be a theory with less data than the molecular density of a brick here on earth. The theory of Quantum Physics has suggested that the brick is made up of constant moving particles which are made up of an infinite number of smaller and smaller particles. This make final proof of anything impossible. Of course not all scientists agree.
Why is it that people with absolutely no training or understanding of a subject feel compelled to comment? Buddy's entire life is dependent on the application of current scientific knowledge. Fortunately we don't need the approval of the uninformed to have our society work to the benefit of all....
Yes, there are many theories in science that are based on scientists' observations of nature; however, this does not mean that nothing is known for certain in science. There are many things that have been proven to be true in science. For instance, it is proven that the genetic material in cells is DNA (deoxyribonucleic acid) and not protein (which was previously the more popular idea due to the fact that proteins are highly complex).