How can I make failed fudge (hard and crumbly)?
I confess to having never actually successfully made fudge (lazy, and until recently the local Wegmans sold it), but it sounds like you want your sugar to recrystallize. Normally that's strenuously avoided, but:


Required: Start with a traditional recipe, not one of the "easy fudge" recipes. The traditional ones start with heating a mainly-sugar mixture to soft ball stage (≈238°F). I found one site with a bunch (that I can't personally vouch for, but am tempted to try): Old Tyme Fudge Recipes.
Try taking the sugar past the soft ball stage. Maybe to firm ball, around 245°F. (This is a mistake I can easily see your mom having done, maybe because she didn't have a good candy thermometer).
Corn syrup is used in fudge (and caramel) recipes to help prevent crystallization. Use plain sugar instead. Same with acids like lemon juice in caramel (and presumably fudge); those should be omitted or substituted with water.
All those admonishments about not stirring once the sugar has dissolved? Probably best ignored. Any tricks about applying the lid to help dissolve any sugar on the side of the pan? Wouldn't bother.
Butter isn't known as a health food. More importantly, fat in the recipe helps prevent crystallization, so why not remove some? I can't bring myself to recommend margarine (or, worse, "spread"), but if you were to substitute, I expect spectacular failure. And of course you are using skim milk, right?
You could try lazyness, I mean all that stirring is tiring! (See "never successfully made fudge", above).
Worst case, brute force it: as the mixture cools, during that time when you're not supposed to stir, grab a teaspoon of sugar and stir it in. 


I'm sure with a little experimenting, a touch of laziness, and some optimally bad recipes, you too can fail at fudge making. The hard part is going to be finding the exact texture you're going for—failure can range from a slight failure of "somewhat gritty" to a spectacular failure of "I think that's a jawbreaker."