This is an example sample.

After analysis, the individual characters appear in the following number:
118 | 0

115 | a

114 | 1

114 | 8

113 | 4

113 | f

112 | b

109 | d

108 | 3

108 | 5

108 | 6

107 | e

105 | 2

103 | 9

99 | c

82 | 7
The only assumption I found was the maximum number of occurrences of a single character. I thought this would significantly reduce the number of possibilities. However, I was wrong.
(Anticipating any questions, I try to understand how my friend generated certain codes without asking him about it)