I have read about dual citizenship/2 passport threads, but need to clarify one aspect. My Thai partner has dual Canadian/Thai citizenship (allowed in Canada) and Thai and Canadian passports. When using Thai passport to leave at immigration, would it be ok to show Canadian passport if asked about visa to Canada? Thanks.

I believe that is standard procedure. Thai passport for entrance and departure Thailand, and foreign passport for the rest...

In any case, immigration will probably not be the one asking for the Canadian passport, likely it will be the boarding staff of the airline, making sure your friend has right of entry in his destination country.

My question is if its ok to show Thai immigration the Canadian passport as well as the Thai passport being used for departure. Anyone had any experience with this?

If asked that is what you would do but normally only the airline will be asking.

Agree with Lop, Thai immigration are never (ok rarely) interested in where you are going.

The airline on the other hand will want to ensure you can enter at your destination, if they ask show them