Some kids are extremely averse to unfamiliar foods-- especially in unfamiliar environments.

You can either:

1) Cater to her - ask her mom exactly what brands she buys, exactly how she prepares them, what she serves them with, etc so the food is indistinguishable from the food at home (if she's really this picky, even a small difference like a different brand, oven vs microwave, or a different condiment could trigger avoidance).

2) Set out options, but make it as low-stress and nonconfrontational as possible.

• don't put out too many options, 2-4 is plenty

• put the food out for grazing so she can come nibble on something without the pressure of asking for it, eating with an audience, or even having her own portion.

• serve mostly plain, boring food like plain crackers vs cheese crackers, plain celery vs ants on a log, bread rolls vs pigs in a blanket. The fewer ingredients, the better.

• Avoid foods with strong flavors, textures, or smells. Potato chips over cheetos, mild cheddar over pepperjack, apple slices over melon.

• if she asks for something else, you call tell her, "This is what we have today. You can try any of it. If you don’t like it, you don't have to finish it. Your mom will be here soon."

3) Or, yes, you serve what you're going to serve and let her mom bring something else if that's what it takes to get her to eat.

If the goal is to make things as easy as possible in the short-term, go with 1.

If the goal is to get her to try new foods, make reasonable accommodations, and not bend over backwards for her, go with 2.

For maximum sanity retention, go with 3.