These are certainly creative, but they are not things the spells are capable of.
Both Frostbite and Ray of Frost specifically 1) target a creature, and 2) cause damage. They do not create large, strong, solid quantities of ice or frost, let alone free-standing or shapeable quantities.
To do that, your player's character need to learn a different spell, because spells do literally only what they say they do.
Your player is definitely demonstrating creativity and I cringe to see that crushed, but how they're trying to bend/break these spells is diametrically the opposite of how the overall game functions. They are effectively trying to be more powerful than they've earned, nullifying the need to grow, earn XP, and learn newer, more powerful spells that do the new things they want to do. They're basically giving themselves power and treasure (which is what new spells are).
There are games that work this way — where you can narrate creative portrayals of magical effects and have them just happen — but those games aren't D&D 5e and they work differently, with different abilities and limitations permitting and gamifying those kinds of creative freedoms. They still have limits, but they're in different parts of how the game works — and D&D 5e for obvious reasons lacks limits in those places, and so has no built-in way to prevent freeform magic from taking over and nullifying large parts of the game if it's added to the PCs' abilities.
D&D 5e is a game about looking at your (limited) personal abilities and resources, looking at the (limiting) environment, and figuring out clever combinations of them in order to overcome those limitations and achieve your goals. Freeform creative magic upends that, and will make the rest of the game kinda break, as its basic motivation loop of striving against challenges, to gain new abilities, and using new abilities to overcome greater challenges will no longer loop.
(That said, you might have fun with that anyway — there is a long, long tradition of bending/breaking games until they are more like the game everyone in the group wishes they were playing — but it only works if everyone in your group wants the game to be that different game. If you're going to let this player run wild with their magic, pause and pay attention to how the other players are feeling, positive or negative, about this change from what they thought they were playing.)
The full description of the Fabricate spell:
You convert raw materials into products of the same material. For example, you can fabricate a wooden bridge from a clump of trees, a rope from a patch of hemp, and clothes from flax or wool.
Choose raw materials that you can see within range. You can fabricate a Large or smaller object (contained within a 10-foot cube, or eight connected 5-foot cubes), given a sufficient quantity of raw material. If you are working with metal, stone, or another mineral substance, however, the fabricated object can be no larger than Medium (contained within a single 5-foot cube). The quality of objects made by the spell is commensurate with the quality of the raw materials.
Creatures or magic items can't be created or transmuted by this spell. You also can't use it to create items that ordinarily require a high degree of craftsmanship, such as jewelry, weapons, glass, or armor, unless you have proficiency with the type of artisan's tools used to craft such objects.
These are all the limitations of the spell.
Walls are objects (like most parts of the environment that aren't creatures), so a part of a wall or a piece of flooring (e.g. a wooden plank) would be considered an object as well. Depending on the size of the chunk of wall, the material it's made out of, and the level of craftsmanship making such a wall piece would require, Fabricate might be able to make it.
In your particular example, it's a stone wall, and thus you would only be able to make a Medium-size piece of wall (that fits within a single 5-foot cube) with one casting of the spell. Your DM would also need to rule that it doesn't require a high degree of craftsmanship to make. And of course you'll need the stone in some form (whether in rubble or from a boulder or something), since that's the raw material that is being magically converted into the new form.
Also, keep in mind that nothing about the Fabricate spell attaches the object you create to anything else. Even if you meet the above conditions, you'd need some way to place the chunk of new wall you've made into the hole in the wall, and to secure it there.
Best Answer
5E D&D doesn't have a strict definition of the word "raw," but the examples given in the first paragraph of the spell description appear to set some clear parameters. The raw materials listed are all in a totally natural, unworked state. Although there's an example of fabricating "a wooden bridge from a clump of trees," there's nothing along the lines of fabricating a bridge from a pile of lumber.
If we want to be safe and take a conservative reading of the spell, then river bed clay is a great candidate for a raw material, but newly crafted bricks (and used bricks from a broken house) don't seem to be in the spirit of the spell as worded. Once that clay got baked into bricks, it's no longer "raw" in any conventional sense.
I could see a DM taking "raw" in a somewhat metaphorical sense and allowing the spell to affect what we would more rigorously term "unworked" material. This would stretch the terms of the spell quite a bit, but wouldn't seem to be grossly out of line with the flavor or power level of the spell.
But stretching the spell so far as to "turn someone's fortifications into a staircase leading right to them" can't really be defended. Those bricks have been baked, assembled, mortared together. There's nothing raw about them. If the spell had been intended for that type of transmutation, it would simply refer to "materials."