He doesn't need to keep a separate username.
But if he wants to be free of upset that his GF accesses his OKC ... he could keep THAT separate by taking action.
- He could tell his GF to not access it and that he expects her to honor his preference even though it is public access and she's a member of the public.
- He could take steps to block whoever and limit his OKC acct accessibility and not rely on her word.
- Do both the above.
- Do something else like decide to not have an OKC.
Basically it's his account he runs it how he wants... but if how he runs it fails to meet his needs for privacy? That's his lookout.