Here's what's really going on, I suspect: the complainers are upset that they had to go to the forced team lunch while your friend (for legitimate business reasons) wasn't stuck there. And because it was a legitimate business reason, they can't really complain about that, hence the annoyance is bleeding over to something else.
I think Yvaine hit it right on the head here.
POD. It's not like friend simply took her lunch and went back to her desk, to avoid being with the rest of the office. She did have a legitimate reason, a business call, which came in when she wasn't quite expecting it. And if she didn't know how long it would take, I don't see any issue with her bringing her lunch back, so she could eat while she took care of business.
And, since everyone paid for their own lunch, I'm guessing each ordered what they wanted, rather than AD simply ordering 10 lunches, 3 turkey, 3 ham, etc. and letting everyone choose what they wanted. So friend took the lunch SHE chose, so how was she to know that they were one short.
AD should have made sure it was all therre at the time of delivery, and if not, called and had another one sent over quickly.