Is it possible to come into Costco to confer with the optical department once, without paying for a membership?

Yes

You can visit Costco with a friend who has a membership card

I believe that only members can.

Only a member

From my experience with costco and several friends that worked there... the only area you can access 100% of the time as a non member is the pharmacy. Frequently mentioned by others is alcohol. (But thats not universal)) But all other departments and accessibility is determined by your location's general manager's discretion. You can always call and ask...or get a member to take you or split a membership with someone... chances are here in MA they'd probably let you slide if you ask..but you'd need that membership access for ordering a pair of glasses

Yes but once you place an order you have to be a member