Is it possible to forfeit a non-vested account balance when the participant terminates rather than waiting until a distribution is requested or until a break in service occcurs. The document states "immediately upon distribution". Immediately upon termination is not an option. Is it even allowed?

Yes. a plan document can provide that a participant with a vested percentage of zero will be deemed to be cashed out. This allows the forfeiture of unvested accrued benefits to occur immediately at severance of employment

If the participant is partially vested, can it still be done? Also, does it require an amendment to the plan?

Nope not if partially vested. Forfeiture in that case happens the earlier of the year in which the participant takes a distribution or after five one year breaks in service.

Am thinking it is 411a6A to 411a6D as the cite. Could be wrong.

You may want to rethink what is being done. Keep in mind that a person can have his or her balance restored if he or she returns to work. Did the person even have a Break in Service? If no, and the person returns to work, you may have a problem if you move forward with "immediate forfeiture". There is a reason why plan language typical requires at least a One Year Break in Service.

Our approach when designing plans always uses the "5 Year or Payout" that Janet M refers to. While I can't say this method is required, I think any other way has too much potential for problems.

Just offering "food for thought".