Haven’t read the whole thread so not sure if these tips have been mentioned.

Silk soy vanilla yogurt is the best. We add our own fruit because the fruit yogurts just don’t taste as good.

Annie’s vegan Mac and cheese is better by miles than other dairy free options.

Oath and Mod are good options for eating pizza out.

Get an ice cream maker and google dairy free ice cream recipes. Almond milk works best for this and is the best milk substitute in most recipes.

Become fans of Asian food.

Dairy foods are calorie dense. Your teen dairy free may need additional mini-meals. My dairy allergy teen now adult used stir Fry's as a quick go to as well as eggs and veggies with hummus.

My kid drinks lactaid milk and we buy lactose free ice cream. We tried a bunch of yogurts and settled on a silk soy yogurt she likes best. I think lactaid also makes a sour cream but we've never tried it. She also eats goat cheese and some sheep milk cheeses like manchego. Vegan cheeses are generally really bad. Your kid may also find that some things bother them more/less than others. My kid is normally okay eating pizza - the mozzarella doesn't seem to bother her and she occasionally eats cheese sticks with no problems. When she's out and she wants something with a lot of dairy she uses lactaid pills.