This is an interesting case. My thoughts:

1) It is cheating, but not a serious offense

2) It is bad etiquette

It is cheating because the player is using a computer to count. Counting is an important part of the endgame and a skill players try to develop, so I think it's fair to say that "ending" the game to see what KGS spits out as the final score and making a decision based on that is akin to using a program to estimate territory earlier in the endgame, which seems to me to be a clear case of cheating. Not a big deal, but a kind of cheating.

That this is bad etiquette, or at least very close to bad etiquette, is, I think, clearer. Some people are more open to having their opponents try desperate invasions late in the game, and it's probably the mark of good character to be able to shrug off this sort of thing and call it a learning experience, but it is nonetheless probably impolite. The boundary depends on who is playing, but I think the extreme example of the player who keeps playing stones that will clearly be captured to encourage the other player to resign out of frustration suggests that there's a point at which this kind of play would become infuriating to even the most patient and open player. Anyway, I'm sure this has been discussed in more detail elsewhere.