Any supplemental exercises other than chip-ups must be chosen very carefully so as to not interfere with progress on [the main 5 lifts].", and "If progress is being made on the primary exercises, you are getting stronger and your objective is being accomplished. If in doubt, leave it out.
So, you're friend is correct, in general, but not about chin-ups/pull-ups. Chin-ups/pull-ups are the only accessory lifts that are explicitly programmed into Starting Strength (3rd Edition).
If you can't do many chin-ups, your press and bench press will increase as you get stronger on this very important exercise. And that is why it is the only ancillary exercise included in the novice program.
Rippetoe also has a lot of good things to say about dips: good substitute for bench during injury, the best exercise for targeting lower pecs and triceps (better than decline bench), better than push-ups because they can be weighted. It's basically the closed kinetic chain partner of the bench press.
You bought a very useful piece of equipment.