The moment
Thank her for something she actually did: the calm way she welcomed a newcomer, the extra chair, the soup, or the story that made everyone laugh.
A useful detail
Do not wait for perfectly phrased gratitude. Send it on the way home while the evening is still bright.
Keep it human
If it is late, send it late. Warmth has no expiration date.
Let the person be the point.
A gift is optional; a sales link never replaces a useful gesture.