This is your friend, your friends parents or your parents friend who did the cooking? If one of my friends sabatoged me that way, I agree, s/he wouldn't be my friend anymore (I would doubt they ever were honestly), but if it's one of the other two it might be trickier.
My suggestion would definitely be not to eat there anymore. If that's unavoidable (because this is a family "friend" and you have to go) eat first. This person will definitely never make a veggie meal. She went out of her way to insure there was nothing vegetarian there. Who HONESTLY doesn't have ranch salad dressing or Italian or Olive oil?? At this point you shouldn't give one wit about this persons feelings being hurt as she is not giving the same consideration to you.
In the future if your friend is the one cooking, you can offer to bring a dish. I don't feel comfortable asking someone to prepare me a special dish but I do usually say "Do you mind potluck?" in an excited voice. It's a nice non confrontational way to insure you have something you can eat. It also shows people there is a veggie life beyond salad.
If it's your friends parents who will do the cooking, you can ask your friend to talk to them ahead of time and mention your vegetarianism.
If it's a friend of your parents either eat ahead of time or ask your parents to ask them about bringing a dish along.
I'm sorry but I would've been a heck of a lot less polite in that situation. I probably would've passed on the whole dinner and, if possible, left early so I could eat at a decent time. I agree you should be gracious to those who try to make you something and you should have more respect for yourself. That person is not your friend by a long shot. She relished "tricking" you into eating something against your ethics and friends simply do not do that. You deserve better friends than that.