I'll agree with you that the mixed powder (i assure you based on your pic there are two different powders there) can not be proven to be the culprit. But we also can not be 100% certain that what you got from him are the same exact loads from the same exact run of reloads that he was shooting, either unless he happens to have a few left and was willing to pull one apart to verify. Even if it did end up having the same mixed powder, i would not be willing to ask him to fire another round out of his gun to see if it doesn't blow up!
I'm still assuming you have a savage. My experience is they have a tight chamber and reasonably short throat. Going off memory, i had to keep my loads around spec length to avoid jamming the bullet into the lands. If the rounds you have are the same recipe that your buddy had been firing and he's not looking at pieces of scope scattered on the ground and a banana-peeled barrel, it is possibly due to a looser chamber and barrel and just plain dumb luck that he is not in your predicament.
Bottom line, the round that was in the chamber at the time your rifle went kaboom was one of your buddys reloads and it was the first one at that and it was charged with an unknown assortment of mixed powder. Sure, it's still speculation, but the evidence is quite overwhelming.
Like i said, take their offer to buy another at a discount if the offer didn't get any better. It's the only way to absorb some loss.
I feel for you man, i really do. This was not a critical err in judgment on your part to accept reloads from a friend, just really, really bad luck...
By the way, what is the OAL of his reloads that he gave you? Just wondering if they were loaded longer than spec based on his experience with HIS rifle.
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