Delemental said:
As Mahali said, in the rules as written the person with the item creation feat pays the XP cost.
Actually, I do not think this is quite accurate. In a cooperative item creation project, the person who expends the xp is considered "the creator" for purposes of the item's Caster Level, and any creator-specific prerequisites (such as "creator must have 5 ranks in Jump"). But any of the co-creators can supply the feat, so long as one of them does.
The problem with the fighter lies in the fact that he doesn't have a Caster Level. Therefore he cannot provide the xp for an item, since that would make him "the creator" and give the item a Caster Level of 0 (or " - " depending on how you interpret it), which is too low for any kind of item creation at all.
If you as the DM want to allow this, then I'd suggest you allow the fighter to purchase/find/obtain an item that allows him to transfer some of his own xp into a "pool" which the wizard can then use for item creation. To prevent abuse, the provider of the xp needs to be willing, and I'd cap the amount of xp that can be "pooled" for later use.
For instance, you might let the fighter hear a rumor of a
Bottle of Essence, being offered by a merchant in a nearby village. Anyone who breaths into the bottle can (if willing) offer up some of their life force (up to [some amount] xp), which is captured in the bottle. A wizard with at least 5 ranks in Spellcraft and Knowledge (Arcana) can then use this stored essence to fulfill the xp requirement in the creation of magic items.
Let the fighter get the bottle (perhaps in exchange for some favor to the merchant, never pass up a good plot hook.

) and then have the wizard pay the standard costs for item creation, but use the fighter's xp instead of his own, if the fighter so wishes.