This is true for some of the subjects, especially math. The rigor in AP Calc AB and BC is more difficult than their counterpart IB Math courses. IB Math courses follow an International curriculum where the various subjects are integrated with each other. So it's not uncommon to study a chapter with concepts rooted in algebra, followed by some geometry, and see some statistics and probability mixed in with math. In the US, math is taught in silos which prevents the student from learning how the various subjects work together. However, the one advantage is that the AP curriculum allows students to dive deeper into math.
Also, the IB HL science classes can be seen as more rigorous than most of the lower-level AP sciences. However, AP Physics C is widely considered the most rigorous Physics course.
For the rest of the course, it is pretty much a wash to say one is more difficult than the other. IB courses tend to work more in cooperation with each other while the AP courses are taught independently from one another.
The most important element is that your child is taking rigorous courses that meet both their abilities and academic goals.