There are two different definitions of "isogram": this uses the definition "there are no duplicate characters"

isogram.c

#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int compare_chars(const void* ap, const void *bp) {
  char a = *((char*) ap);
  char b = *((char*) bp);

  if (a > b) { return  1; }
  if (a < b) { return -1; }

  return 0;
}

bool is_isogram(char *word) {
  for (size_t i = 0; i < strlen(word); i++) {
    word[i] = (char) tolower(word[i]);
  }

  qsort(word, strlen(word), sizeof(char), *compare_chars);

  for (size_t i = 1; i < strlen(word); i ++) {
    if (word[i-1] == word[i]) {
      return false;
    }
  }

  return true;
}

int main(void) {
  printf("enter a string to check: \n");
  char *line = NULL;
  size_t n = 0;
  getline(&line, &n, stdin);

  char delim = ' ';
  char *tok = strtok(line, &delim);

  while(tok != NULL) {
    char *copy = strdup(tok);
    if(is_isogram(copy)) {
      printf("'%s' is an isogram.\n", tok);
      return EXIT_SUCCESS;
    }
    free(copy);

    tok = strtok(NULL, &delim);
  }
  free(line);

  return EXIT_FAILURE;
}