Proper administration of improper Libero replacement

[Scrapper1]

NCAA rules, please.

In yesterday's match, I was R1. Receiving team wins rally. Since Libero was in the left back position, she correctly leaves the court and the proper player replaces her by entering in the left front position.

However, the Libero then re-enters the game into the center back position. My R2 (a 1st year official, who has a lot of playing and coaching experience) does not notice, and apparently the Libero Tracker doesn't say anything.

I delayed my beckon, hoping that the table would figure it out, but they didn't. So my question is, how should I have handled this situation? Do I call my R2 to the stand? Do I simply blow the whistle, and show a TDW? (Then call the captain over to explain?) Do I beckon for serve and call a rotation fault?

For transparency, I called the R2 to the stand and asked him to confirm the illegal replacement. When we confirmed it, I issued a TDW and did not allow the Libero to remain on the court. (We brought the original center back player back into the game.)

Thoughts? Suggestions? How should I actually have handled it. Thanks.

(If it's different in FED, I'd be interested to know that, too; since we've gone to NFHS rules for HS in MA.)