I was chatting with a friend the other day who told me that she is now certified to dive to 40 meters
...
Then she produced a PADI Adventurer card
Here are the PADI
Guidelines:
Open Water: 20 meters/60 feet
Advanced Open Water: 30 meters/100 feet
Deep Diver Specialty: 40 meters/130 feet (Recreational Diving Limit)
Now if you accept these to mean "Is Certified to Dive This Deep" then your friend would need to have the Deep Diver Certification for the 40 meter limit.
The prerequisites for Deep Diver are:
1) PADI (or equivalent) Adventure Diver Certification Level (Open Water plus 3 "Adventure Dives" with an instructor, an Adventure Dive is the first dive of any PADI Specialty Course) or above.
2) Be 15 years of age or older.
Maybe what happened is that she has her 3 Adventure Dives and thus the C-Card with one of them being Deep Dive #1 which would make sense if she's heading toward Advanced Open Water Certification.
This does not equate to having the Deep Diver Certification which required 4 dives and a separate book with material not included in the "Adventures in Diving" Manual.
I have the Deep Diver Certification. Some of the additional skills for the Deep Diver Certification are:
Using a Redundant Air Source (in my case Pony Bottle but using instructor's Octo also qualifies) for ascent & safety stop, extended safety stop, compass navigation at depth in excess of 20 meters/60 feet.
Tell her to complete 2 more adventure dives being sure that Deep & Navigation are in her list of 5 then she'll be "Certified to 30 meters" and if she really wants to be "Certified to 40 meters" then it's just 3 more deep dives with an instructor for the Deep Diver Certification.
Of you can explain that the agencies just provide guidelines and as a responsible diver she should remember to always dive within her limits & training. When she want to expand either there are many instructors & experienced mentors who will help her safely & gradually expand both.