What are ways for graduate students to make additional money? A friend of mine started a PhD program after finishing his diploma with the best possible grade.

Unfortunately the institute does not have a lot of resources so he is being paid from a project fund for 3 years for a part time job.

He needs the full time to work on his PhD thesis, however, so works half the time for free.

This and his friends starting to work in the industry for considerably more money made him feel left behind, questioning his choices and that he is being taken advantage of.

The project that is funding him is capped at 3 years, so he doesn't think he could work part time to supplement his income.

Are there any ways, scholarships for example, for these kinds of situations, or does my friend simply need to reconsider his choices and quit his PhD to go into industry as well.

The field is aero-space engineering.
Finish the PhD quickly.  People with PhDs are paid more than people without PhDs.  Becoming a PhD faster will get you to higher pay faster.  You will also keep that pay for longer, because you will work more years of your life if you finish your education sooner.  (note:  correlation between education and pay does not always imply causation; also in certain disciplines the pay is terrible both with and without a PhD)
Quitting the PhD to go into industry is a good choice if making money in the short term is the goal.
Switch to a university which provides decent funding.  This works best if you are early in your PhD.
Apply for outside fellowships.  However, keep in mind none of these pay that great.
Working a job unrelated to the PhD while enrolled will make the PhD take longer, delaying the time when you are paid more.  Do not do this if you can avoid it.