While looking at another post on this topic, I followed some links and came up with this:
M.C.L. 257.710d. Child restraint system required
(2) This section does not apply to any child being nursed.
O.R.C. § 4511.81. Child restraint system required
(F) If a person who is NOT A RESIDENT of this state is charged with a violation of division (A) or (B) of this section and DOES NOT PROVE to the court, by a preponderance of the evidence, that the person's use or NONUSE of a child restraint system was in accordance with the law of the state of which the person is a RESIDENT, the court shall impose the fine levied by division (H)(2) of section 4511.99 of the Revised Code.
Damn. She might get away with it, unless Ohio law has anti-dumbass provisions. I would hope that Michigan passes a law providing that the exemption only applies if the persion doing the breastfeeding isn't driving.
Update: Well, she's not as protected as it first appeared. Not only is she a resident of Pennsylvania, she also has no driver's license anywhere. Being that it's Ohio, they might throw her under the jail.