Is the 50 pounds the raw weight, or does your friend want 50 pounds of servable brisket?
Are you planning on cooking whole brisket or flats? It's good you are confident with the brisket cooking process, that is the hard part. Reheating and serving is the easy part. I'm assuming you will be onsite to oversee the reheating and most importantly the slicing?
I generally freeze "cook ahead briskets", this way I can do them a few weeks in advance. Since you know how to cook a brisket, I'll skip ahead in the process starting with the wrapped step:
My brisket is on the smoker with an internal around 160° and some nice color. At this point I opt to do a wrapped finish with a beefy au jus to cook it tender, a resting step where I allow the internal temp to drop to 145°, a quick chill in an ice slurry, a freezing step, a thawing step, then a reheat step, and finally slicing.
- I double wrap the brisket (fat cap up) adding a cup of beefy au jus and cook it tender, then remove from the smoker, vent the foil and let it slowly cool to 150° then re-seal the foil pouch.
- I have a 100 Qt cooler with an ice slurry (20# of ice and water) ready for chilling.
- Put the wrapped brisket in a 2.5 gallon zipper bag and plunge it in the ice (keeping the foil seam up). Your goal is to get the brisket below 40° as fast as possible. This might take 45 minutes.
- Meanwhile, I make a triple layer foil "boat" which holds the brisket and liquid, but it's not large enough to be sealed, and prepare a triple foil wrap large enough to hold the boat and be sealed. Brisket goes in the boat fat side up, along with all the foil drippings, plus some additional au jus. The boat is placed on the second tripple layer of foil and the pouch is sealed.
- The pouch goes into the freezer.
- For thawing, the brisket is slow thawed in the refrigerator for 48 hours+ depending on weight.
- For reheating, the foil pouch goes on a tray and into a 250° oven and reheated to 145° - 150° over the next 90 to 120 minutes. Pouches can be held in a hot box.
- For slicing, the foil liquid is reserved and de-fatted, and the brisket is sliced when guests begin to move through the serving line. I like to slice for plating, not slice ahead and hold in aj jus.
Sometimes I don't attend the particular event I cook for, so I mark the brisket slicing direction when they are raw. And I send along a quart or so of additional au jus along with an instruction sheet that summarizes the smoking process, and has full reheat and serving instructions. If you decide to go this route, shoot me a private message with your email, and I'll send the instruction sheet.