what age?
avoiding milk canbe tricky (it is a "hidden" ingredient in many many foods) but it is possible.
does your friend want them to have a milk substitute? there are various soya/rice milks available but they have their cons as well as pros.
calcium is a concern (how much of a concern depends on their age) but it is possible to find ways around this - either by substituting for a different "milk" (the soya ones are often fortified, for example) or by supplementing with vits/minerals.
you can even get around that by extra calcium rich veg etc - opinions differ on how much calcium is vital to our diets anyway.
free from sections in major supermarkets will have products to fill most gaps, but do be aware that they contain a lot of filler substances too, and are not always that healthy.
cooking form scratch and knowing what goes into foods is often the best way to be sure about stuff.
reading labels is the way to go, along with knowign what you are looking for - eg whey powder, butterfat etc.
if the twins are very small, and not yet weaned, her doc should be able to advise on a hypoallergenic formula for them - I don't know about these, sorry, as dd1 went dairy free from the age of 2 (and by the time I had dd2, I kind of knew what I was doing, was able to breastfeed, and so never needed them)