[i] He hadn’t better tell them. (CGEL, p.113)
[ii] He had better not tell her. (CGEL, p.196)
According to CGEL, negation can be marked after or before better. But I can’t find any examples of [i] in COCA or BNC. So it seems to be not grammatical issue but others in there. Why aren’t there any examples of [i] in those corpuses?