The attacker should roll a Stealth check with advantage.
The burrowing monster is hiding (under the ground) and heavily obscured (because the ground is in the way). Perception checks to detect a heavily obscured creature have disadvantage.. You can either let the burrower roll Stealth with advantage or give the victim -5 on their passive Perception to detect the ambush. If the burrower wins, they'll have surprise during their attack.
Note that, surprise or not, the burrower and the guy on the surface still can't see each other until the burrower comes out of the ground.
The attack will have neither advantage nor disadvantage.
Depending on the monster's anatomy, either it can attack before digging through the surface (say by stabbing a big stinger or horn up out of the ground) or it can't. If it can't, then it has to come up first, and then the attack is resolved normally.
If it can attack while burrowed, then it's still locating its victim by tremorsense alone, which is not enough to aim accurately*, but the victim also can't see the attack coming. As usual when the attacker and defender can't see each other, advantage and disadvantage cancel out. This is technically different from the above case in that it also negates other sources of advantage on either side.
The question has been raised: is tremorsense accurate enough to aim an attack? I think it's not. The rule here is that "When you attack a target that you can't see, you have disadvantage on the attack roll."
Tremorsense doesn't say that it lets the creature see (like Darkvision and Truesight), nor that the creature doesn't need to see (like Blindsight). It's also not mentioned in the Vision and Light rules, a strong clue that it's not a kind of sight.