You are lucky that she expressed such an apathetic response to your "confessopn."
The next girl might call the police.
Why Punky? Masturbation is not a crime.
The next woman might respond positively and he he will end up with a sexting partner. There are women who enjoy sexting. As long as he doesn't persist with a woman who doesn't respond in kind, there is no harm in initiating this sort of thing.
He initiated a sexual text. Her response showed she wasn't interested. He continued to talk to her in a non-sexual way. She was OK with that. Life goes on. I don't think our OP did anything wrong.