gingy, I don't have enough info but I think that perhaps your SO's exDIL (now that's convoluted!

) might feel that while he's interested, he's not actually ready. The parenting board I referred to is rife with stories of kids who refuse to wear anything but underwear, BUT are not actually potty trained, so the parents are just going around cleaning up after them all the time.
I'm pretty happy with how it's gone for us. I realized that according to the criteria in this article, the sozlet has been "potty trained" for a very long time, in terms of going in the potty often enough that diaper use was cut down significantly. But I consider "potty trained" to be wandering around in underwear and clothes, and then either a) going to the potty herself, successfully removing underwear and clothes first or b) telling me that she has to go, and successfully waiting until I've helped her on the potty (at a store for example) and helped her with her clothes.
At any rate, it's been a smooth process (knock on wood), she is proud of herself, we're proud of her, there aren't any big guilt or power things going on.