Refuse entry

There is no surprise to me why they banned her to enter. Don’t you know that if a person is a frequent traveler to Singapore then he or she must have strong reason to come here. What does she have strong reason to be here, other than meet one person (to be noted ICA doesn’t recognise bf gf kind of relationship, on which someone could visit here regularly).

If you want she should be here, then go to her country, marry her then bring her as a legal wife under LTVP.

There are two possibilities why she was denbied entry: Either they suspected her of illegal work, or of (ab-)using the travel visa to in fact reside in Singapore.
As Surya said, anyone staying longer (or more frequent) than the average visitor (who stays 3.5 days in the country) should have a good reason for it. Joining a boyfriend (i.e. trying to establich residence) is NOT a good reason in their eyes.
I recommend she only visits for short durations (up to two weeks or so) once every three  months to avoid such suspicions!