@rush99999The spirits are never too preoccupied! ;P
More seriously, spellcasting is fairly second-nature for her. She was grinding EXP in magic skills for hundreds of years, longer than most humans have been alive, and got pretty good at it before she was done in. Besides, the ring-tightening spell is one of the core functions built in to the ring. Eru was never under any illusions about the complex nature of the people wielding her rings, and even considered the possibility that there could be wearers other than the ones she intended. The tightening spell is a binary on-off feature, turned on when the ring is worn by an enemy or by someone inclined to reject tough love, and turned off in most other circumstances.
The follow-up spell that she was setting up for doesn't have to happen immediately.