Mortal sin requires three elements: grave matter, full knowledge, and full and free consent. If any one of these three elements is missing, there is no mortal sin. Based on that, wouldn’t it be true that there is essentially no such thing as mortal sin for anyone other than Catholics? Can’t one argue that, since he isn’t Catholic, his church doesn’t teach mortal sin and therefore he did not have full knowledge?
Just because a person’s church does not teach about mortal sin does not mean it does not exist at all. Analogously, just because a non-Catholic Christian church does not believe in purgatory does not mean that its members may not end up there one day. So, yes, it is possible that a non-Catholic can commit a mortal sin.
That said, only the person himself and God can determine an individual’s personal culpability for an objectively grave action. Only God can determine whether that person dies in his friendship. Thus, while we can say that it is possible for non-Catholics to commit mortal sin, we cannot make any presumptions about anyone’s personal culpability or eternal destiny.