I'm not Michelle, so I can't answer for her.
But I'll share my food allergies experience. Twice I've had reactions similar to what your DIL experienced. One time, after eating at a mom 'n pop Mexican restaurant, I got itchy hives all over my body, and my face, lips, nose, and ears swelled. I went straight to the doctor, and was given prednisone injections and pills to take for a week. The doctor said it was probably an allergy to a preservative, which is hard to test for.
Five years later, I ate an EL Fudge cookie and had the same reaction. I was told by my advice nurse to go straight to the ER. This time I ended up being admitted to the hospital overnight. When I was discharged, I was given a referral to an allergist and given a battery of skin-scratch tests. I tested postive for being allergic to shellfish, lamb, and rye. I never really ate any of those products anyway, so it wasn't very helpful. The allergist didn't test for preservatives, and I was told that they are really hard to diagnose anyway. I still stay away from EL Fudge cookies, shellfish, lamb, and rye anyway. Since then, I've never had a bad reaction to anything I ate, either. (Knock on Wood!)
Food allergies are to be taken seriously, and if anything similar happens to your DIL again, tell her to go straight away for medical treatment. If she thinks she's allergic to beef, try to avoid it at all costs. She might also ask her doctor if she could take Tagamet (the antacid) if she thinks she's having another reaction. My doctor ended up prescribing it to me just in case. It turns out that Tagamet has antihistamine properties, and won't generally cause drowsiness like most other antihistamines.
My doctor said that when reactions like your DIL's and mine happen, it's an outward manifestation of what's also going on inside the body. So all that swelling that you see in her face and neck is also taking place internally, and could seriously affect her breathing.
I hope she gets a definite answer soon, but not because she has another reaction that requires medical attention. Good luck to her!