I’m doing some travelling with my mate in a few weeks and I was wondering if, to save time, he could come through the Schengen border through the EU Line with me even though he is not a citizen? I’m a dual Australian and Italian citizen, and he is an Australian citizen and has a 1 year residency visa (Italy).
Generally not. Non-EU citizens who are family members of EU citizens within the meaning of the free movement directive are entitled to use the EU passport lines at Schengen borders, but other traveling companions are not (see articles 10(2) and 2(5) of the Schengen Borders Code).
You can of course accompany him in the "all passports" queue. While that won't save any time, it should at least be less boring and/or more convivial.
With a residency visa and accompanied by you it is worth a try, but no guarantees.
I've had this several times in Germany with my girlfriend, who has a non-european citizenship, but a residence permit. The first time we entered Germany together, I stood with her the "all passports" queue and when we finally came to the counter, the officer looked at us and asked why we hadn't gone to the "EU" line. Which after this we did and never had issues.
This may not be official policy, however.
A study details the differences of member states treatment of unmarried partnerships. For example, about Germany it writes:
"Germany [...] does not include common spouse partnerships and does therefore not seem to recognise the principle of durable relationship."