The xp system works like this (from what I understand from reading the posts).
Player 1 and player 2 both create new PCs on day 1, both play this PC exclusively and often until day 30, but each PC is of course different. Player 1's PC is a blood seeking barbarian that only wants to smash things, they gain most of their xp through combat. Player 2's PC is a scholar that only goes into dangerous situations to learn about the area, and as such spends more time rping but still goes to dungeon from time to time. Both these characters starting at the same time, should still gain the same amount of xp over the 30 days, because as Player 1 is constantly grinding dungeons, eventually they enter exp penalties on combat xp (and it is combat xp only from my understanding), and while player 2 is not gaining as much combat xp, they never reach these exp penalties because they do not constantly dungeon. Thus despite player 1 being in blind drive, and player 2 being in possible bonus xp, both characters should be at (or close to) the same level after 30 days.
It is irrelevant to either player if the other player stops playing because as stated above, the XP you gain is not based on another player's progression. So if either player stops playing that character is has no effect on the other player. The comparison between the two becomes voided at that time, the system just works in a way to prevent two people that play equal amounts of time that also started at the same time to not fall behind another character based on the rp style of that character, ie. Dungeon grinders, nor rp focused PCs gain xp faster than the other. (As long as the rp focused PCs do actually dungeon).