Inviting a friend to a restaurant, on a friday, is not enabling.  Or condoning.  Or sinning in any way.  The person who was invited has free will and will eat what they want, and neither you nor even God will interfere in their decision.  Sure, you could stop going to lunch on fridays, if the activity is habitual.  But 99% of the population does not avoid meat on fridays, so how is this activity even surprising?

Thanks for all the feedback.

a) Standard suburban place, big menu with lots of non-meat options. Time (Friday or not) and place (diner or fish shack or steakhouse) is pretty much out of my control with this particular person. I offer to pay every nth time or so since I'm usually the one dining for free.

Sometimes cajoling or distraction works, sometimes not. When it doesn't, given the mix of temperaments and history involved, my options are either let it slide or get up and leave. The latter won't fix it since I'm hoping and praying this person returns to the Faith before it's too late. 
Whereas staying silent might be the lesser evil double effect strategy (nod to Ladislaus).... or not either, since the sin committed is certain and immediate, while the reversion is uncertain and far off. (We need a sigh emoji.)

b) Change of plans, bbq is getting dropped off on Saturday, albeit an Ember Day this week. I had thought that Trinitarian Baptism binds any Christian after the age of reason, my error. But that's a whole other kettle of fish.

It is possible -- although improbable -- that a person has been canonically exempted from Friday penance, e.g., an episcopal exemption (temporary or perpetual) or one may have paid the annual La Cruzada fee in Spain.

or one may have paid the annual La Cruzada fee in Spain.

No, since the times of Pius XI (i think, at least it was the case under Pius XII) the bull was made to be valid for only one year, and it was renovated every year. Montini never renovated it, so... no one can claim to use it.
Also, that benefit of the bull was free for poor people, poor meaning that they need to work to sustain themselves.

While not entirely on point with the O.P., when it comes to Fridays and someone asks us over to eat for dinner, I follow the advice of St. Paul in 1 Corinthians 10:27: "If any of them that believe not, invite you, and you will be willing to go; eat of any thing that is set before you, asking no question for conscience' sake."

Granted, I know full well that my truly Catholic friends will be serving us a meatless meal.

This is not a proper application of St Paul's advice, who was speaking of the situation of eating foods sacrificed to pagan idols.  The council of Jerusalem forbid Christians from eating such foods (which is obvious) but this command was not absolute; there were exceptions.  Such as, the avoidance of such foods were to avoid scandal to the newly converted, who may be confused between Catholic rules and their former life.  Also, this rule was only for Jerusalem and surrounding areas, which is where the main scandal was probable.

There are also exceptions for the Church's law forbidding the eating of meat on fridays, but typically such exceptions relate to sickness or health of the individual.  Scandal has never been an exception, and the rule of law is not a local one, but applies to the entire Church, as many saints say this law is of Apostolic origin, and thus part of Tradition (which cannot be changed).

So, I think your thought-process on this matter is slightly flawed.  If one were invited to a meal by an unbeliever, who served meat, I'm not sure if one could partake (assuming pre-V2 rules), for charity's sake.  I'm sure there are situations were it would be allowed, but the point being that the "Friday Fast" is more strict of a law than the "Avoid Pagan foods" law which St Paul refers to above.

Technically speaking, Catholics do not absolutely need to abstain on Friday if they perform some other penance in its place.  

Are you sure about that?  Traditionally, the Friday abstinence was mandatory (making allowance for exceptions such as Jone describes in his moral theology text), and substituting another penance wasn't an option.  

Here are the exceptions that Jone cites:

Are you sure about that?  Traditionally, the Friday abstinence was mandatory (making allowance for exceptions such as Jone describes in his moral theology text), and substituting another penance wasn't an option. 

Here are the exceptions that Jone cites:

True, I’m going by the novus ordo.  Is it valid?   I’m not the Pope.  (and doubt the current occupant is, either!)  In some situation under pre-V2 and even pre-1955, a person could get a dispensation that could even his entire household.  
I’ll leave the research to you, but I’m very confident of this.  
Pardon the indelicacy, but our entire family got a dispensation from an SSPX priest. Our Dad, head of the household, had to do the preparation for a medical procedure on a Friday.  It involved the drinking of large quantities of gelatin based liquid!  (For those who don’t know, gelatin is derived from bone marrow. You can buy vegetable based gelatin in the supermarket, but the medical product offered no choice other than lemon-lime or orange flavor!)  
Of course, there was no need for anyone else to use the dispensation.  Poor Dad was limited to his choice of jello or clear broth.