This is the advice that many will give you: if he doesn't have one already, get your friend a teacher right away. The double bass is similar in tuning, but not in technique. Improper DB technique will result in injury.
That said, the teacher will probably have a preferred method book to work out of. Some examples are
Simandl's "New Method for the Double Bass",
Bille's "New Method for Double Bass" for example. I provide links to Lemur music, but there are many other places to buy as well.
Here , and
here and
here and
here , for example, are other threads discussing beginning method books. Searching the forums will turn up lots of info.
