I assume romanDigitChar returns one char for an exact matching numbers e.g. 1, 5, 10, 50, 100 etc only. printRomanNumeral would call this repeatedly for known values as numbers to turn them into characters. I suggest two nested loops, one for amounts with specific characters in decreasing value and one extracting each value. The inner loop calls the second method.
I assume he/she expects ASCII characters although there are special Unicode characters for Roman numerals.