It really depends on her and what she feels that she can/wants to do. I am assuming that she got into a relationship with you ex knowing that he comes as a package with kids, therefore when your DS is with her, she should treat him as she would her own child. So unless she has some major issues I don't see the problem with her taking him to his lessons. In fact, it would be a bigger problem if she refused to IMO.

It's your child, so of course you care, but unless you have a real good reason not to trust your ex's new partner, then it's just one of those things that you will have to let go....