I have a family member from Malaysia who will be staying with us for 3 months. Before I contact the Insurance Company (Metromile), I want to know if I can add him to the car insurance temporarily, what is normally the best advice to allow him to drive, and in case of an accident we are covered?
Regarding driver license it depends on the single states you will drive in. International Driver License is sometimes not recognized by the law, so your friend's usual license is enough.
For example in California:
The State of California does not recognize an International Driving Permit (IDP) as a valid driver license (DL). California does recognize a valid driver license that is issued by a foreign jurisdiction (country, state, territory) of which the license holder is a resident.
quote from dmv.ca.gov
Regarding the insurance I think the best way to go is to contact the insurance company and ask, for sure you will find much better and precise information than on a forum.