If person A truly wants to move off in a different direction, take the stock and run. Don't waste your time and energy on a lawsuit, particularly a venture that has no capital. Put it into your new direction.
If you really, absolutely can't get it out of your craw, wait until B's venture is funded and then sue.
If you really, absolutely can't get it out of your craw, wait until B's venture is funded and then sue.