The execution of this stunt as shown is legal. However, if the flyer goes inverted as she travels around the waist of the base then there must be a spot who is able to spot the flyer and protect the head. They flyer must be able to protect the head all the way around the stunt.
If the top person is, in fact, inverted, then the person under her cannot be in a position to protect in order to be consistent with other rulings. If that base stood up, for example, and the top person was totally upside down on their backs, we’d clearly say they weren’t in a position to protect. Changing the degree of inversion shouldn’t change that call, as we’d really be into different interpretations at that point.