That is an informally correct use of ‘platonic’, but not too common. You can talk about a platonic meal or outing to mean that it wasn’t with the intention of exploring your romantic compatibility. But I don’t think I’d ever say either ‘platonic’ or ‘friendly’ in the invitation itself. We went out for what I thought were platonic coffees, but now I realize that he likes me.

Platonic is used in terms of friendship and relationships suggesting there is no romance or intimacy involved. Not entirely sure what you mean by your question…. You mean if you take a friend ( of your sexual preferences) out for a coffee but it is not a date invitation? You can say it is just a coffee, nothing more…

I just say coffee, as long as both parties are aware it's platonic, only saying coffee is fine.

I just say coffee. They probably know that it's only platonic because of our friendship. But if you really need to be specific, I would say "a casual cup of coffee" Causal meaning not serious, just to have a little fun and some conversation.

"Platonic coffee" would not mean non-romantic coffee (whatever that means). It would be coffee that does not come from real beans. It would be coffee that was born in the imagination. It would be coffee that could not be drunk except in dreams. "Coffee" is already a friendly word. You can ask anybody to meet you for a coffee in a public place. "Platonic" does not only mean "non-romantic". You are thinking of "Platonic love" or "Platonic relationships". However, the word "Platonic" is broader than that. "Platonic" ideas are spiritual, non corporeal, ones.