I don't think so, cuz if that person was alone he would still be seeing stuff which would force that into reality, reinforcing him into reality cuz hes the one observing it. Idk though wave particle duality and the uncertainty theory and all that stuff is confusing
What I think you're referring to is heisenberg's uncertainty principle, which(I think) basically states that you can never be fully certain of an object's position without observing it. That is not to say, necessarily, that the object doesn't exist if it isn't observed.
You're assuming matter, must be obseved to exist. One would be encouraged to ask if this in and of itself is a problem .
Whoever told you that matter only exists if one observes it is extremely incorrect. Take the invisible man for instance. If he were to knock over a book shelf whilst invisible, then he would obviously exist since an object at rest remains at rest unless acted upon by an external force. BTW, invisibility is only an optical illusion. All you do is bend light around an object, and poof, it's invisible.