That's a good question and one that I have no idea what the RAW answer would be.
The closest parallel would be Bull Rush, where by default the character just stops when they can no longer move. Sort of like actually punching someone back a few feet - the hit's gonna hurt bad but tumbling into a wall or other person isn't really. The real question is whether or not the forced movement causes AoO's. The rules for Bullrush seem to imply that forced movement normally causes an AoO because they go out of the way to prohibit it with a Bullrush, but it's ambiguous.
The closest parallel would be Bull Rush, where by default the character just stops when they can no longer move. Sort of like actually punching someone back a few feet - the hit's gonna hurt bad but tumbling into a wall or other person isn't really. The real question is whether or not the forced movement causes AoO's. The rules for Bullrush seem to imply that forced movement normally causes an AoO because they go out of the way to prohibit it with a Bullrush, but it's ambiguous.
Isnt involuntary movement do not cause AoO unless specific feat (Greater Bull Rush) is used?
Generally all involuntary movement doesn't provoke without a feat (such as Greater Reposition, Greater Bullrush, etc).
None of those feats can apply to this no?
Let's check the feat wording to confirm this:
You receive a +2 bonus on checks made to bull rush a foe. This bonus stacks with the bonus granted by Improved Bull Rush. Whenever you bull rush an opponent, his movement provokes attacks of opportunity from all of your allies (but not you).
The feat specifically calls out the Bull Rush maneuver, which Force Punch does not say you do. There are spells that allow you to perform a Bull Rush maneuver, and those spells would be affected by Greater Bull Rush. (It would make for an interesting caster character build, that's for sure.)
As for what happens, who knows, the spell is ambiguous on these matters. We could treat it as if they were falling (which means 0 damage), or we can find another ability which accomplishes a similar effect and see if it applies. For example, Telekinesis:
Weapons cause standard damage (with no Strength bonus; note that arrows or bolts deal damage as daggers of their size when used in this manner). Other objects cause damage ranging from 1 point per 25 pounds (for less dangerous objects) to 1d6 points of damage per 25 pounds (for hard, dense objects). Objects and creatures that miss their target land in a square adjacent to the target.
Creatures who fall within the weight capacity of the spell can be hurled, but they are allowed Will saves (and spell resistance) to negate the effect, as are those whose held possessions are targeted by the spell.
If a telekinesed creature is hurled against a solid surface, it takes damage as if it had fallen 10 feet (1d6 points).
So we have a way that can be plausibly ran. The affected creature (or to be more accurate, a dead body, for the most relevant example) is treated as a hard, dense object, and deals 1D6 points of damage per 25 pounds it weighs to the friendly flanker. I'd also rule that the affected creature would also be taking an extra 1D6 damage, since he's being thrown at a creature (which is technically speaking a solid object).
I don't personally think the first part would make sense, and would give the Force Punch ability a level of power that it shouldn't possess as a 3rd level spell, but I think having the second part apply to both creatures would be the most rules-sensible as far as RAI is concerned.