Assuming you're limited to just potions, the very best you can do is a CL 15 potion of cure serious wounds. That would heal for 3d8+15, and cost 2250g. This is assuming you can find a level 15 caster to brew them for you.
However, if he has an alchemist friend with the Infusion discovery, and his ability allows him to drink extracts, then the best he could do is a CL 15 Heal extract, which would heal for 150. I'm not sure how you would price such an extract, but if you use the potion pricing rules, it would cost 4500g.
Cooperative Crafting increases the value of what you are crafting per day, not the amount of things being crafted in any given time, nor the amount of time you take to create a potion/scroll. Valet familiars gain the feat even if they do not meet the requirements.
If your crafting limit was 1,000 per day, with a valet familiar, this limit is now 2,000 per day. But what the feat helps with is the Crafting Progress you make each day. If you would take 10 days to craft a 10,000 gp magic sword, with a valet familiar helping you, this task now takes 5 days.
For potions and scrolls, that are normally 250 gp per 2-hours work, it doesn't become 500 gp per 2-hours, but means that your daily progress will be limited at 2,000 gp. Which makes the feat only useful for higher level scrolls or potions crafted using a higher level caster level, as potions are capped at 3rd level.
You cannot craft two items at once, nor you can craft twice the amount of potions. You are still limited by other rules:
Potions and scrolls take 2 hours per 250 gp on their price, or 1 day per 2,000 gp on their price.
Other magic items take 1 day per 2,000 gp on their price.
The GM could house-rule that it also increases the 2-hour limit for potions and scrolls, but that's not what says on the feat.
Ultimate Wilderness adds the (optional) rule to collect Trophies from defeated enemies, which are parts of their bodies that have some value, either as decoration or for their mystical properties. The latter actually discounting the creation of magic items that have affinity with the trophy:
For instance, devil's hearts can be used to create magic items that require spells with the [evil] descriptor, giving you a discount. The heart of an Osyluth (CR 9, 10 HD) is worth 850 gp in the market, and for magic items that require an [evil] spell, it would be worth 1020 gp instead when used as magical supply to create magic items.
The Monster Hunter Handbook introduces another way to harvest enemies, using the Harvest Parts feat.
While those are slightly less valuable, you are not restricted by the creature's affinities. You are also limited to 25% of the item's cost using those supplies.
Using the (optional) rules for Downtime, you may obtain capital which can be used to craft magic items. Those are abstract magical supplies that can be used to all sort of things, not only crafting, and can be flavored as "gathered herbs to create potions".
Each Magic capital is worth 100 gp, and to Earn them it costs 50 gp (half the Purchase price). And they may be utilized in crafting magic items just as if they were magical supplies.
You may also gather herbs as Goods (20 gp to buy, 10 gp to earn), and later convert them to Magic:
Keep in mind that this could potentially reduce the cost of creating magic items in half, as it costs half the capital value to earn them, with the only downside being that it could take much longer than simply purchasing the capital or magical supplies. Talk with your GM if it's okay to do that, as some GM's aren't keen on methods that increase the party's wealth.