Shirk accidentally by swearing

My mother was talking on the phone and I heard her say “I swear to my kids” I asked her about it and she said she knows it’s haram but she just wanted the person on the phone to believe her. I told her it was shirk and she said “Astaghfurillah, God forgive me.” Does it count as shirk towards her even if she didn't know? And should she do Tawbah?