In our family, DH disciplined DD#1 just like he did our two. Occasional spankings (used when things needed to make a big impression, always flathanded on bottom, never more than a couple swats), groundings, removing of property...all were done with my blessing. He has never treated her differently than his two; games, doctors appointments, school shopping...all the same; always told people he had 2 daughters (and 1 son). I would say the only thing that he really did different was straight on alone time after she turned 12, but that was due to her moving in with her other parent and his mommy. Her other parent did not discipline her at all (so it's no wonder that when she turned 12, she asked to move with him and his mommy and the courts said she was old enough to make that decision) and his occasional girlfriend or the 2nd wife did nothing to her either. If any of the gf or the now ex 2nd wife had been inclined to parent her, I would have asked to meet with them and set boundaries, just as I asked other parent to do when DH and I got serious (but he never did, so it fell to me and DH to decide what was appropriate).
In your case, OP, you would need to talk to your ex about the SM disciplining your child in that manner, and if you find it did happen and you have a problem with that, then you need to handle it with your ex and SM.