Recipes for which you can substitute margarine for butter can have non-dairy margarines substituted instead - you have to check the ingredients list, most margarines contain milk solids.
Here in Australia, my dairy free friends and I use Nuttelex, but that's not available in the USA.
cooking.stackexchange.com/users/10066/Freya
In general lard can always be substituted for butter, but you will not get the same flavour, obviously.
Lard is perhaps a bit more 'savoury' than butter, so it may be worth trying a pie out just to see if it works with a sweet filling.
You should be able to convert straight from butter to lard.
Another alternative is vegetable shortening, which you can use in all kinds of baking.
cooking.stackexchange.com/users/4194/ElendilTheTall
If you use Butter Flavored Crisco its an ' acceptable ' substitute for butter in a 1:1 ratio.
cooking.stackexchange.com/users/6279/Cos Callis
Coconut oil is gorgeous in crusts and many baked goods.
Get the best quality raw you are willing to pay for and she won't miss butter at all.
Try a bottle of macadamia oil too.
A little in baking where butteriness is wanted or even sauteeing veg.
Supposedly, these oils are back in favor nutritionally so baby will get the benefits too.
cooking.stackexchange.com/users/6638/Pat Sommer
Although most people have said that you can do a 1:1 substitution, I want to warn you that it's not always true.
The problem is that butter is part water, and so when you bake with it, it'll give off a little steam.
If you use it melted it'll add water to the mix, which will develop a little bit of gluten when mixed with flour (first recipe, part of the second).
You may need to adjust your recipe slightly.
Typically, you want to reduce the amount of lard or other solid fat used by about 10-20%, and add back in 10-15% water (or other liquid; if you're dealing with un-melted butter, you may want to use vodka so it won't add to gluten development, but will still add steam).
That being said, one alternative might be to use clarified butter or ghee.
Depending on how sensitive the person is, it might not be worth the risk, but for people with a milk protein intolerance who really crave the butter flavor they might be willing to try it.
(and, as with the solid fat substitution, you need to adjust as it's all oil, no water)
cooking.stackexchange.com/users/67/Joe
If you use lard or criso, you'll want to substitute canola or another neutral oil for a portion.
Butter has a lower melting point than lard or shortening, so a little oil help match the mouth feel of butter.
I can't help you on the flavor.
Maybe sprinkle some buttered popcorn jelly-bellys on top?
cooking.stackexchange.com/users/11082/Grant270
My wife is currently unable to consume any dairy as our breast-fed baby has a milk protein allergy. When reading many dessert recipes, many of them call for butter. For example:  http://smittenkitchen.com/2008/04/shaker-lemon-pie/ or  http://chefmichaelsmith.com/Recipe/cinnamon-rolls/   I know pie crusts for example can be made with either, but can they always be substituted?  When would I not want to use lard over butter?  What ratios should I use to convert the recipe?  Is there some other milk-protein free alternative that would work better?
Lard (or other fat) as a subsitute for butter in desserts for someone with a dairy allergy?
cooking.stackexchange.com
<substitutions><dessert><allergy>
cooking.stackexchange.com/questions/24833
cooking.stackexchange.com/users/231/talon8
Recipes that allow margerine instead of butter can be substituted with non-dairy margerine. Lard and shortening can often be used instead of butter, but a little oil can be added to match the mouth feel. Coconut oil, macadamia oil and ghee or clarified butter are also good options. You can try a non-dairy margarine, lard, coconut oil, macadamia oil or ghee in baking.
Try a non-dairy margarine. You can always substitute lard for butter. Add a little oil to match the mouth feel of butter. Try coconut oil, macadamia or ghee.