Teeth slenderizing is normal with Invisalign
It's normal to slenderize teeth for Invisalign, but giving out other patients' names is not done or normal. It's called doctor-patient confidentiality. There may be testimonials on his or others web sites regarding this treatment, but those people have given their approval to be listed.
It can be necessary to slenderize the teeth during Orthodontic treatment
Both Invisalign and Braces move or change the position of the teeth. In order to do this, in may cases (especially extreme crowding) we need to make more space giving the teeth room to be guided into proper alignment. We call this "Slenderizing" or "Interproximal Reduction" (IPR). Teeth have a memory and are always in movement. Retainers prevent the teeth from shifting back into their former position.
There are a few options that can be discussed with your Dentist. Regular retainers are acrylic with some metal in them and eventually are worn only at night. Invisalign offers clear retainers. A fixed or permanent retainer can also be placed on the lingual (tongue side) of the teeth. It is a small thin wire that is bonded in place keeping the front/anterior teeth in place, for some patients this can work very well because it is bonded and always in place.
In my office I have before and after pictures of satisfied patients who are very happy with the resultsof Invisalign. (and other procedures) For the patients who want to do this we have them sign a release form that allows us to display their pictures and testimonials. Invisalign has a patient case portfolio on the website that can be viewed as well.
Sometimes IPR cannot be avoided
Slenderizing the teeth often makes the case work, where it otherwise would not, or would be very difficult. We aren't talking a TON of enamel, just "enough."
If the other patients have not agreed to divulge their feelings, the dentist likely CAN'T share their thoughts. It might be a good idea for him to get a few patients opinions to have on hand to share. If you are asking for phone numbers to call and discuss, then I don't know if anyone would be OK with that.
IPR may be necessary; Patient confidentiality may be an issue
Filing, or interproximal reduction, may be required to create space required to allow the teeth to move. It is common with any kind of orthodontic treatment, not only Invisalign. The aligners themselves are needed to move the teeth.
There could be many reasons for your dentist not showing work from or directing you to previous patients. In most jurisdictions, health care professionals are not free to divulge health records. Unless there is a signed consent, the dentist is actually working within the ethical standards. That being said, do your research before committing to a provider.
Best of luck!