Sandollars Seashells

I’ve heard of crushed seashells for aquarium snails, but what about sand dollars?

Are crushed sandollars the same thing? (litte skeletons)
And will this harm a betta in the same tank?

Similar enough to work. If your water is soft and acidic, it could cause a problem for your betta by dissolving too quickly and raising the pH too fast. It will increase the pH, how much and how fast depends on how soft your water is. The softer it is, the faster it will dissolve and the quicker it will become hard with a higher pH.
If your water is soft, put a little in one day, a little more the next and so on. You will need to be religiously faithful to your water changes, too. Big changes will change the pH closer to tap pH, and too big of a change could shock your betta.

She sells seashells, down by the seashore