Interesting Bony Hands Combat Question

[Stygian Inquirer]

Hey guys, I was just DMing an encounter between a Medium-sized Monk and 4 Medium Bony Hands (VRGttWD) and he was surrounded by the 4 Medium Bony Hands like so:

X = Bony Hands
O = Monk

---X
-XOX
---X

Because of the Bony Hands' Partial Invisibility, the monk failed his spot check and thus couldn't see them and he tried to tumble out of them. How would you deal with someone tumbling out from being surrounded by people he cannot see?

[vipera aspis]

treat the ground as uneven terrain?

[Drinnik Shoehorn]

Increase the DC?

[ScS of the Fraternity]

Attacks of oppertunity - if the tumbler doesn't know that they are there, then he/she can't use tumble to avoid the attacks of oppertunity.
I would rule that the boney-hands have the option to not take the attack but to force the monk back into the square.

[Undead Cabbage]

I'm wondering exactly how the monk could logically tumble out of this situation without an attack of opportunity? Did the player justify how the character would tumble out?

Otherwise, this looks to be similar to the typical case where a player insists on making a hide check in a well light room with no cover or shadows.