Almost thread necromancy, dear Yuri. Depending on mods.
Use your [Hex Spell] Vulnerability on them, and they they are now vulnerable to Hexes more than once... including [Hex Spell] Hex Vulnerability itself. The only thing you lose out on is CL to overcome SR and Saves, neither of which matters for a beneficial Hex since your ally will simply forgo their saves.
Meaning, so long as you keep hitting them with [Hex Spell] Hex Vulnerability and never let it drop, you're good to go for the whole dang day of infinite beneficial hexes.
If it ever DOES drop, just be sure to keep Hex Vulnerability prepared as a 1st Level Spell to get the gravy train rolling again. Downside of course is the low Duration of 1 rd/level, meaning it requires rather frequent refreshing.
Or you could just Cackle non-stop.