-What I means is that
If she leaves US before the RP is approval, she needs to come back to US before her stay in China reaches two years continuously even if the RP does not expire. - Sorry but I don't see it and I don't think so. Actually she is fine as long as RP being valid. Looks to me you might have reversed the reason/result I guess.
If she leaves US after the RP is approved, of course she has to come back before the RP expires. Obviously, she wants to leave US before the RP is approved.