So practical suggestions from a string teacher (and yes, I play cello as well as fiddle):
If he really wants to do this:
1) Arrange main bag to be transferred each day, and walk with the cello. Do not trust the cello to the baggage transfer - even if there is no ill intent, it will likely be chucked into a van under a pile of other bags.
2) Bring a cheap, student instrument, not a good cello.
3) From everyday life and walking ca 45min each way with a cello on my back fairly regularly in the last couple of years, I strongly recommend the backpack attachment from BAM. This is cushioned, and has a proper waist/hip strap in the same way as a regular hiking backpack would (with a small pocket that you can put stuff in). From experience, this fits onto non-BAM cases but may take a bit of wangling. If you need a four-point attachment, the Gewa equivalent may work but is less ergonomically friendly.
Honestly, I occasionally tuck a plastic recorder into my backpack but having considered bringing my fiddle I came to the conclusion that it wouldn't be a good idea - if I want to pick up some Galician tunes I'll head over to the area especially for that, and I can always record some things on my phone if I want to take them back. The chances of playing every night are very slim.
Also, heads up, you'd be paying an extra seat on the flight(s) if they even allow the instrument to travel, and may have difficulty with trains and coaches.