In neither A nor B did you sin, nor did you encourage sin, so what they ordered is not your fault.  That's on them.

 Had you not taken them to lunch on Friday, they would have had lunch somewhere else and eaten meat anyway. 

Re A. knowing what was known about the person, the host knew there was a probability/possibility of the person not abstaining  nor fasting that day.  Therefore the host provided the temptation to sin, and that's on the host.

You knew this person was/is a Catholic.  You just provided an occasion of sin for them.  Take them out on another day.  What they do on Friday alone is on them but you put the temptation before them.
It was probably a good guess that they'd order meat.  Why not take them to a seafood restaurant if it's Friday?

Provocation/temptation doesn’t apply because the temptation comes from the person himself (ie he’s hungry).  It’s not like she asked the person “Do you want steak for lunch?” Nor does it sound like she invited the person, on a Friday, to a steakhouse (and then ordered shrimp).
Of course she did. not a steakhouse but a full menu establishment.

The silence aspect only applies if you are a superior of the sinner (then you have an obligation to correct).  The person in question did NOT order meat, thus they were not silent on the matter, but followed the law.  Also, you don’t know that the person didn’t say say, “It’s Friday, I’m ordering fish.  What are you getting?”  Which would be a reminder of the law.
 In all the places the "Nine ways of being an accessory to another's sin" is published, nowhere is it ever stated that these nine ways apply only if you are a superior of the sinner.

The rules of fraternal correction apply here.  One is not obligated to go out of their way to stop evil (if that we’re even possible, due to free will), especially if the person will reject your attempts and become more bitter against God/Church.  If you repeatedly scold/preach to sinners, (and you’re not a superior), you actually commit a sin yourself, against prudence and charity.
This hardly qualifies as going "out of their way" .

Ps you’re using the word “host” incorrectly.  An invitation to a lunch place is not the same as “hosting” someone at your house and grilling a steak on Friday.
 No I'm not.  The OP stated she was paying therefore the other person was being invited as her guest, thereby making her the host of that guest.  Check the dictionary.  Location/venue is irrelevant.

Your brothel example is dumb because it’s a specific place, which is an occasion to sin.  In regards to restaurants, 99.99999% of them serve meat of some kind, every single day.  Meat is not an occasion to sin but it’s just forbidden on fridays. 

And, if we want to get technical, V2 changed the law and allowed meat to be eaten on Friday (if another penance is substituted), so outside of Ash Wed or Good Friday, there’s no more meatless Fridays.  You can’t be sure the person sinned because they could’ve done a private penance later.
OP only asked for thoughts on the limited scenario given so I only gave an opinion on what was provided

Quote from: FarmerWife 5/22/2024, 10:07:57 AM
That’s ridiculous. I haven’t had a problem getting a meatless option at a restaurant, there are vegetable soups, salads, probably fish/lentil/bean. The person is not a child.
I don't understand your point.  It was a full menu restaurant as obviously the OP didn't order meat.  I think everyone understood the person wasn't a child.

No, sorry, you’re wrong.  Only if the person had made it clear (ie “Hey, if you take me out to dinner on Friday, I’m ordering meat.”) then you’d be guilty. 
No, with the info given in the OP scenario the host knew there was a very good chance the guest wouldn't abstain.

Putting this out there to see what folks here think/do. Say you have one of these scenarios or something similar:

a) You take an apostate Catholic out to lunch (this is a close relative) saying you'll pay. It's a Friday. This person knows the precepts of the Church from childhood but orders a meat entree anyway out of habit.

b) A Protestant friend asks you to bring back (and fronts you the cash for) a few pounds of really good barbecue since you're traveling anyway near a town that's known for really good barbecue. Given the timing, odds are that this will get eaten on a Friday.

Both of these people know about your traditional Catholic dietary habits, have commented that these are "weird", but will ask about and listen to catechesis respectfully enough.

I figure that a basic principle is not to enable other peoples' sins even if they don't think they're sinning.

ABSOLUTELY !