kitten, thanks for catching this. I realised my silly error too after posting and also that I hadn't mentioned pork and lamb. Rarely encounter lamb, but pork gives me same reaction as beef (perhaps lamb does too, unbeknownst to me).

30-year vegetarian here. My view:
1. yes, ingesting beef products makes me ill. I noticed this within 5 years of becoming vegetarian. I can also tell by taste (it's subtle, but distinctive, like the taste is 'off' in a noticeable way) if ingredients include beef products, such as in soup bases or crackers made with lard. That said, tiny amounts will not be enough to cause illness, so perhaps the amount of lard in the refried beans was so small that it didn't bother your friend. For me, I've had some nausea in the past from quantities such as soup broths made with meat. My sense is that something in one's body chemistry changes and is not able to tolerate beef after a few years as vegetarian.
2. I've not had any problem with chicken products (like unknowingly eating chicken-based broths, for example). Different chemistry.
3. No illness problems at all with fish products, even fish itself. I don't eat fish normally, but for a short period about a decade ago, whilst traveling in Korea (where it was very difficult to find/eat enough dairy or other non-meat protein), I did eat a few meals with fish in them for the pragmatic reason that I really needed some protein. No illness at all - again, different chemistry.
4. It's uncommon for hosts to make entirely separate dishes for vegetarian guests. Being a dinner guest, even at many restaurants, can be discouraging for vegetarians and others with diet and/or allergy restrictions, and, depending on the attitude of the host, deeply alienating (you'd be amazed how callous, rude and passive/aggressive-hostile some hosts can be about this!). Makes you feel like you're imposing on your host and would be better if you weren't there. My point - I'm sure your friend was really touched and appreciative that you made a special version of the dish just for her. That in itself is such a wonderful thing that I suggest you definitely *not* tell her about the trace amounts of lard, as it was unintentional on your part and evidently not enough lard to cause her any problems at all. Not worth ruining the happiness and pleasure for her of a friend who cared enough to make a special vegetarian dish for her. Compliments to you!

The Brussels waffles are delicious, also the crepes. I've been back several times just for the waffles. Very authentic recipe, not too sweet, clear flavour of eggs in the batter, excellent. Best are those made by the owner (who is French) - he's there most weekends, some weekdays. Great cafe ambience too.

Just discovered a small Moroccan place named Argan near Rittenhouse Square (on 17th, just up from Walnut). Opened about a month ago - it's the real deal. Menu is modest for now, sandwiches and salads, clearly items meant to look familiar and non-threatening to Americans. But the dishes and flavours are authentic and delicious. I had their Zaaluk sandwich (roasted eggplant), excellent. Very nice young couple on staff, seem to be the owners. I hope they do very well in this neighbourhood with its big lunchtime crowd. Really looking forward to my next sandwich there.