Which would you have MarieTeresa do?
First, don’t make a public fuss, and don’t present a teaching during the Q&A period.
Most of us (Protestants and non-Catholics) are conditioned through years of public school to
believe the teacher - MarieTeresa isn’t going to accomplish anything by heckling, no matter how calmly, reasonably, and unemotionally she heckles.
From the RCIA participant’s point of view: It’s the
teacher who’s grading us, and who’s opinion determines whether we move up to the next level, or not; not whomever else, no matter how eloquently they may speak.
Second, using the channels provided by the Church, either correct (best option) or (if correction is impossible) get rid of the Catechist.
A second teaching can be given at a later date by an
authorized Catechist (including MarieTeresa herself) which would
then be received as authoritative by the RCIA participants. She could passingly mention the first incident by stating that some members may have been confused by a previous presentation on this subject, but, as far as possible, don’t mention it any further than that.
If the students themselves point out that the previous teaching was totally opposite, acknowledge that fact, but don’t get into the blame game, and don’t call the previous catechist names like “heretic” etc. - even if true, it only muddies the waters. Just say, “Even with the best of intentions, things sometimes come out wrong. We’re just trying to correct any confusion that may have occurred,” and leave it there, while presenting the accurate teaching.
If the entire parish is heterodox, and she can’t improve things in this particular RCIA, then she should take her candidate to a more orthodox parish for RCIA. But again, don’t call names - just say, “We’re changing parishes because I think this one will provide you with a broader variety of Catholic experiences,” - which it probably will.