The assurance from the loadmaster that he replaced a strap and re-tightened everything should have been enough for most flight crews.
This is where I totally disagree. in 16 years of flying cargo, I have dealt with a wide range of loadmasters - from the very good ones to whom I went for advice, to the bad ones whom I could not trust to tighten a strap without checking afterward.
The fact that the crew had little or no experience with such loads should have raised their alert level, and they should have been asking a LOT of questions. A broken strap is NOT a common occurrence. A broken strap on an unusually large & heavy load is a red flag.
To merely replace a broken strap and re-tighten others would have NOT been a sufficient remedy on my airplane. I would have asked to see the load guidelines in the W&B Manual AND asked for additional straps rigged in the direction(s) along which the broken strap was rigged.