hmmm, interesting question indeed
I will assume that the stated ally is neither helpless, incorporeal or ethereal, tiny or smaller or physically absent in any other way while the Spring Attack is taking place. Your question is not phrased all that clearly with regard to this
As for attacking/acting in an occupied square (occupied by whatever, lets hope its not a solid wall ), PHB 3.5 erratad (?) printing page 148 gives some clues.
"You cannot end your movement in the same square as another creature unless it is helpless"
Which, to my mind, would mean, as long as you are in the same square as a non-helpless creature (or those smaller than tiny ), you cannot perform any action that is not an immediate legal movement to leave that square. If you _stop_ moving in an occupied square, you immediately get shunted to the next viable adjacent square (page 148-149, "accidentally ending up in an illegal space". )
Also check the rules for "squeezing" - if 'squeezing' into a space smaller than your required "square size", you cannot attack etc. I would argue, that since a specific square (occupied by another creature) is already "taken", any further creature would have to squeeze into said square. And hence be unable to do anything but move.
As for "Spring Attack" - single or double move : If one employ "Spring Attack" by ruling a (full) move action before the attack and a standard action to move again after the attack, there definitely is no way one can attack from an occupied square, as stopping there is simply not permissible by the rules.
If one sticks to the "single move and attack somewhere during that move" version postulated by Hypersmurf , than I would actually rule similarly, although only on instinct, especially, if the movement takes the form of the common forward+backward attack, and not a "ride by" style of movement in a straight or slightly curved, continous line. The ally is in the way of the attack and blocks your square.
In all honesty, this sounds like a massive abuse of the rules, especially their spirit, either on part of the player or the GM. Give him a good smack over the head Chi Lu, will you ?
Oh, and as for the many uses of "Spring Attack" - its quite wonderful, especially if combined with a monk's or scout's speed enhancement and "skirmish" damage...