If there is a formal custody arrangement, this overriding of a child's consent and of the co-parents' agreement is worth talking to their lawyer about.

If there is not a formal custody arrangement, then no longer allowing the boundary-trampling parent to take the child on solo outings might be worth considering, after a very serious conversation about body boundaries and consent. Sure, you can see kid. But not alone. Just as you'd handle it with any creepy relative you can't cut off entirely for whatever reason.