The adjustable spout on the powderhorn looks like the one Mark B. Burnham invented, and marketed via ads in Muzzle Blasts in the fifties and sixties. My dad had one, and my brother still has it. Last time I saw it, the old horn had a lot of bug damage.
The bag reminds me of those made and sold by Harry Merklee, who also advertised in Muzzle Blasts throughout the sixties and into the early seventies. I had one of his bags, years ago. It had similar "rawhide" binding around the edge of the flap and a horizontal wooden toggle, similar to the one in the photo. If I remember correctly, Harry would add beaded rosettes at extra cost. I don't remember the exact design of the beadwork he used, though. Rosettes similar to those were and still are available from Indian craft/hobbyist suppliers. My pouch had a similar square-bottomed knife sheath with a side flap like that one, also, but my sheath was affixed to the strap. The "patch knife" that was provided by Mr. Merklee was a cheap wood-handled skiving knife, available from Tandy. Your antler-handled knife with the straight-razor blade looks like a home-made or custom job. I think those were popular during that same era (1950's - early 70's), also.
I believe my bag was made of "latigo" leather, alum tanned and heavily oiled when new. Bootlaces cut from it are usually marketed as "rawhide." Probably the most over-rated leather ever sold. It is very prone to mildew, and I recall having to clean my pouch frequently. It corroded the nickel-plated buckles that were on the strap. So, the rot may not be indicative of extreme age. I think in time people became aware of the shortcomings of this type of leather, and moved to the more durable and less corrosive bark tanned leather, now generally marketed as "veg tan." That's probably why your other bags have held up so much better... A different type of leather.
However, that period of time, the 1950's through early 70's, was sort of a "Great Awakening" in muzzleloading. New replacement parts and mass produced replicas of historic firearms began to be available, and more hobbyists were starting to build their own guns. Reenactment, in my opinion, was a very new concept, and the resources we have now, and accessibility to published resources, were simply unavailable at the time. I was a child in the 1950's but became an avid reader of my dad's gun magazines by the time I was in the third grade. Costuming goals for muzzleloading shooters at that time mirrored the outfits seen in Disney's "Davy Crockett." Minnetonka moccasins were perfectly acceptible, judging from photos in the magazines. A lot of the gear and accoutrements from that time look "kitschy" to us now, but the sport we enjoy these days was built on the foundation laid by those folks 50-65 years ago. I think your friend's bag and horn are collectibles in their own right, as representative of that time. You are lucky to have them.
Best regards,
Notchy Bob