Remember, the cohort can be a maximum of two levels below the PC. How is an 11th level character outdoing an entire 13th level party, even with buffs? Are their characters really that poorly built?
Also, do your intelligent recurring foes recognize this level of loyalty? What would the cohort do when the bad guys started targeting the PC - especially an underpowered PC since the player is giving his magics items away to the cohort? Or targeting the cohorts family back home? Heck, this can even lead into some good adventures!
read leader ship"If a cohort gains enough XP to bring it to a level one lower than your level, the cohort does not gain the new level—its new XP total is 1 less than the amount needed to attain the next level." So it can be with in 1 level of you and other party members. Only when recruiting it does it have to be two levels lower then you.
to the op
The problem you are have is not leader ship problem since the PC is using it how it was intended to be used, the problem is a the fact the pc is a caster and focusing all his buffing on the cohort. He is not spreading the love. Spell duration are being abused or you are not giving enough encounters in the day. This is the problem with all casters, and most GM's not give enough encounters in a day to actual use up the casters spells. Especial at level 13th up or spreading them to far a part allowing for 4 or 5 rounds of buffing instead of 1 or 2.
You want to fix the problem try 13 or 15 encounters in the day all spread out through out the day even encounters during camping, Don't give the players time to buff. hit them back to back with 3 or 4 encounters in 30 min time spans then nothing for a hour then again. Combat and spell casting makes a lot of noise and is going to attack a lot of unwanted attention. This may cause your pcs to stealth a bit more meaning they are moving slower and allowing more buffs to wear out before next encounter.
You need to plan the encounters based on where pcs are, such as on the road in the woods, maybe a Troll or two goes to investigate the sounds of battle. Shortly after picking up loot and doing a little healing (you don't want to tpk them, you just want to challenge them and let they all shine) The pc are attacked by the just arriving trolls. It does not have to be hostel creature to make pcs burn spell or resources. maybe they stumble upon a wounded npc party and they ask for aid, and in return they give the pcs a magic item or some gold. This cause the good cleric in the party to burn some spells or channels. thus reducing the number of spell they have in the day. This will allow the other pcs chances to shine and give RP chances.
Another thing how long does it take to count 1,000gp.(Really see how long it take you to count 1,000 pennies) How long does it take to apprise all that treasure, ID those magic items. Start using that in your game. All the buff will start wearing off faster on the pcs, this will allow the other pcs to shine that are not getting buffed all the time it may also make the caster cast less buffs, allowing the cohort to out shine the pcs. look at how long it take off armor, now they want to remove that armor from 5 a dead body as treasure. It take them 5 times a long. The clock is one the biggest tool the GM has don't let player control it for free.