Find out if it is a preference, a mild allergy or a severe allergy.
If it is a severe allergy you'll need to carefully check ingredients of all food and completely avoid cross contamination (i.e not using the safe knife or chopping board if it has been used for something containing milk or gluten). You'll also need to know if the child has an Epi pen and whether or not they are anaphylactic.
You may already know this but dairy-free means milk-free, so no milk or milk products (eg no yogurt, no cream, no butter). People
Who are dairy free can still have eggs as eggs are not dairy (ie they are not made from milk).
To check the ingredients literally read every single one of the ingredients and check any mention of gluten. Checking something is dairy free is not so easy as there are multiple words you need to look for. Look for milk / milk powder / milk proteins / butter / cheese / whey / yogurt / cream. Sometimes the manufacturers list MILK in bold or caps after listing one of the other words, but not all of them do this,so you need to be careful with your checking.
If you have no knowledge of food allergies it is a minefield and if would be easier to ask the parent for help.
Tbh I wouldn't allow another parent to cook for my allergy child unless they did have knowledge of food allergies. It's just too dangerous as my child gets severely ill. Food allergies can be a matter of life and death.