How to print the whole line that contains a specified byte offset in a file?

With GNU awk, keep the number of bytes read so far in a variable, and when it reaches your byte offset print the current line and exit. E.g.:

$ awk -b '{ nb += length + 1 } nb >= 80 { print; exit }' file
eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut

The keyword length is a shorthand for length($0), which returns the length of the current line in bytes (thanks to -b). We need to add 1 to it as awk strips off the line terminator.