Just a few years ago, she wouldn't have had any choice but to give him a life sentence. I think they changed the law in accordance with Miller V. Alabama and applied it to adults as well.
No, state law has (at least to my knowledge) never had a mandatory life sentence for second degree murder, and it certainly hasn't been that way any time recently.
MCL 750.317, which went into effect back in 1931 says that second degree murder "shall be punished by imprisonment in the state prison for life, or any term of years, in the discretion of the court trying the same."
Miller v. Alabama is a completely separate issue.