We (addicts) don't think much about consequences right before a relapse. We think - "I just want that feeling again for right now." or we think, "I can do this tonight and then I can stop". We (mothers) think, "How can you be so stupid, so reckless, so uncaring?". Having been in both pairs of shoes, although I understand exactly the mindset of relapse, it still drives me crazy.
What's good is that she called you, confessed, and especially that she went to her sponsor (if she is, in fact, there). Putting the drug in her body again WILL set off her disease all over again, but a large part of her seems to regret the decision and she just might pull herself out of it before going all the way back down.