Echoing my answer here on another stack;
At the start of Shrek : Forever After we see that Fiona's parents are about to sign their kingdom over to Rumpelstiltskin in return for him using his magic to release Princess Fiona from her curse. The news that Shrek has rescued the Princess literally interrupts the signing ceremony and they tear up the agreement.
Since Prince Charming is attempting to rescue Fiona for the sole purpose of becoming heir to the throne of Far Far Away, if Shrek hadn't rescued Fiona then her parents would have signed the deed, Rumpelstiltskin would become King and there would have been no need for Charming to rescue Fiona which explains his total absence from the film:
King : Nothing is worth more to us than our daughter.
Stiltskin: Just sign it and all your problems will disappear.
Page : Your Highness! The Princess! She's been saved! [by Shrek]
They tear the agreement in half.
Presumably he's still living in Far, Far Away, doing whatever it is that princes do when they're not busy.