Firstly assuming the test is actually being performed correctly and with functioning tester...

If it has NO compression, then the valves are not closing.

For starters whoever rebuilt the head needs a bit of a slap, or certainly their work given a re-check.

Presumably he is doing a compression test because of other symptoms/problems too ?

It could be as simple as a lifter issue, or a complete and utter cock up with poorly installed valves.