If that developer has only ever made two contributions to the project, then obviously that developer isn't in the right mindset to make useful contributions. I find that when I switch to part-time on a project, my performance degrades. Producing good code requires that you understand the existing code REALLY well (and that implies that you have to spend a lot of time just reading it) - If you don't understand the existing code really well, then you cannot produce good work.
One of the worst things I keep witnessing is managers rushing new employees into finishing tasks quickly - This is really bad because new employees don't understand enough about the code to be productive; when you rush them, you essentially force them to implement dirty hacks which will end up costing way more in the medium and long term.
Most managers don't like to pay engineers to lean/understand stuff - They want to pay for code that works. Engineers should be treated more like managers and less like office cattle.
I've seen so many horrible managers in this industry and they always get away with it by blaming the engineers.