The fraud, posing as her "husband", has divorced her & remarried, and therefore maintains no legal standing from which to dictate her fate.
He has reportedly told others that given they were only 25 at the time, of course they never discussed what she would want under the circumstances.
Terry has a loving family that does want, desparately, to take care of her.
When you add to all of this the distinct possiblity that he is responsible for her current state, there is no plausible reason that Michael should have a say in Terry's fate.
Michael refuses to divorce Terri. Rumor has it (anyone got a cite?) that it's because it would be against his fiancées religion. His conduct would cause any divorce-court judge to grant an immediate divorce if Terri had an honest guardian to bring such a motion on her behalf. But she doesn't, so Michael remains her HINO guardian.