Most Helpful Girl
Most Helpful Guy
I would, but make sure in the card you talk about what it is. Have them delivered with a gift letter. Talk about how proud of the children you are and how much you respect her for having such a part in their lives, and for staying with you for 21 years. Tell her exactly what you feel, if there are no ulterior motives that's something anyone reasonable should be able to read in such a letter, even a new SO.
I would accept that personally, if he put it out there in writing.