How does a user become a translator?
I gave him the translator right.
The usual procedure is to create an account, make some test translations from the main page, and wait until an administrator (often it's myself) gives you the right. It's a measure to filter out spam bots and obvious vandals. Occasionally mistakes happen: good translators are rejected because of am administrator's misunderstanding, and occasionally vandals are approved. But most of the time it seems to work well.
Since you told me that you know him, we can skip the test translations step.
If both of you are going to submit translations now, I recommend that you use the same list of projects that I put on your user page, but each of you works on a different item in the list. For example, you can work on "Core MediaWiki" and he can work on "Visual Editor".