Define 'view'.
First, yes, you can 'view' and manipulate, larger atoms more-or-less directly using a scanning-tunneling electron microscope. E.g. this
https://www.physicsforums.com/insights/can-see-atom/
More commonly, you have x-ray diffraction crystallography. If materials were
not composed of atoms, then x-rays would pass through the homogeneous material and leave an even pattern. They do not, they diffract (are deflected) off the atoms in a material. This not only shows the material is made of atoms, but you can also calculate the atomic locations from the diffraction pattern. This is used on a regular basis (as in many times every single day) to figure out the structure of proteins and other complicated molecules. An early application was to find the double-helix structure of DNA.
Second, it's not and never has been, a requisite to be able to observe something directly to 'know' that it exists. If you see dog poop and hear barking there's probably a dog around, even if you haven't seen it.
To begin with, all of chemistry is based on the theory that stuff is made out of molecules, which are formed from atoms, belonging to various elements. That's was pretty much universally accepted by chemists since the Karlsruhe Congress of 1860. There was simply no other way of making sense of chemistry that worked. And that was long before anyone had any idea of what 'atoms' might be made of.
Physicists had universally accepted the idea of atoms as a physical reality (rather than some "chemical" theory) by 1911, with the publication of Jean Perrin's http://www.archive.org/details/atomsjean00perrrich" . If your friend is skeptical, let him read the book that settled the debate.
Or, give your friend a good book on the history of science, or, you could point to the excellent three-part BBC documentary "Atom".
The topic of the existence of atoms hasn't been up for any real debate for a century. You might as well claim the world is flat and that satellite photos are fake.