score:4
The Taiwan Travel Authorization Certificate is documented in Timatic, the system which airlines use to verify that passengers have the correct travel documents.
This is what Timatic currently says:
Visa required, except for A max. stay of 30 days (not applicable to holders of Emergency or Temporary Passports) for nationals of India holding a Travel Authorization Certificate (obtained via https://oa1.immigration.gov.tw/nia_southeast/).
- Applicable to holders of a permanent Residence Permit issued by Japan;
- applicable to holders of a valid resident or visitor visa (incl. electronic) issued by Australia, Canada, Ireland (Rep.), Japan, Korea (Rep.), New Zealand, USA, United Kingdom or a Schengen Member State. Those who hold a work permit are not eligible. Resident card or visa that has expired less than 10 years prior to the date of arrival can still be used for entry.
I cannot tell what it said five months ago. But today it is valid if you have a permanent residence permit, residence permit or visa issued by Japan. Your friend should not be denied boarding today in similar circumstances.