I know I'm late here, but wanted to add my thoughts anyway.
I think a very polite way to tell someone about the food issue is to "politely protest" when someone offers to make a meal for your family. Something like this:
Friend: S., I would really like to make a meal for your family. What day would be best for you?
You: Thank you so much for the offer, but I'm afraid that would be too much trouble because my husband is lactose intolerant.
Friend: No trouble at all... I'm sure I can find something lactose free to make for you.
You: Well, if you're sure you wouldn't mind, we would appreciate that!
That way, you politely mention the issue and give the person an "out" if they want one.