I'm no lawyer (thank God!). With that caveat in mind...
If you are looking for the legality involved, it appears that an important piece of information is missing from the description you provided:
How far along was the pregnancy? Roe vs. Wade determined that a fetus is viable between the 24th and 28th week of a pregnancy. Modern medicine (as opposed to the 1973 medical technology when Roe vs. Wade was made), however, has pushed fetal viability back to the 23rd week. Whether Michigan now recognizes the newer 23 week point, I do not know.
Roe vs. Wade found it constitutional for individual states to ban abortions if the fetus is viable.
This Site says that Michigan has, indeed, banned post-viable abortions.
So with that in mind, if she was under the 23/24 week mark in her pregnancy, then there is the
possibility that no charges could be filed. Reality, however, will probably be different due to the nature of this whole sorry affair.
If, however, she is past that time period and especially if she is near that magic 38 week mark, where it is a fully developed human being capable of living outside of the womb, then Michigan law has (if the above site is correct) banned such abortions, consenting or otherwise.
However, due to the incredibly touchy issue of abortion, people on both sides of the issue will most likely try to push for their "side" to prevail legally.
Other legal factors include:
Michigan's Informed Consent for Abortion Law. This mandates that certain information be made available to women seeking an abortion at least 24 hours prior to the abortion. As silly as it sounds, the failure of the boy wielding the bat
could be added onto any possible charges, just to lenghten his potential jail term.
Possible statutory rape charges if the girl was younger than Michigan's age of consent when she first got pregnant. I believe Michigan is 16. But if the boy got her pregnant when she was 15, he could easily get more charges filed against him.
Parental Notification Laws. I do not know what, if any, Michigan's are in this area. Silly sounding under these circumstances, but
possible such a law could apply.
There is - unfortunately - no law against stupidity.
The obvious answer is stricter bat control laws. Only professional baseball players need baseball bats!
