>A quick check at one service which lists such large amounts, you would be looking at almost $20k/year to keep a single offsite copy of tha
Amazon Glacier would be about $205/year to store 20 TB. A full restore would be like $2,000 though, unless you want to restore 1 GB/month. Still, that's a significant difference from $20k/year.
Storage on Amazon Glacier is $0.01/GB per month, not per year. That's $205/month.
The problem here sounds like the OP is trying to do this on the cheap. I'm surprised that this question even made it to the front page today. "How do I back up data?" is a solved problem. My local Sam's Club has 20TB worth of storage on the shelf that I could have running by lunchtime. Amazon could have twice that amount on my doorstep tomorrow.
Reliable backup of this amount of data will cost more than a $5/month "unlimited" Backblaze or Carbonite account.
Pushing it to a cloud provider is attractive since they would handle storage, disaster planning, electricity, and hardware failures. If they had a 100MB/s connection that they could keep saturated, it would take 3-4 days to push or retrieve the data. This level of service would cost money.
The OP should tell his friend to stop being a cheapskate. If this data is worth saving, it will cost time, money, and planning to protect. Other posters have pointed out the economics of tape vs. hard drives. If the friend thinks this is too expensive, then the data isn't worth saving.