Is there a theme to the party? I made the cakes for my grandmother's 90th and my grandmother-in-law's 90th. They were both elaborate cakes (stacked tiers, kind of like a wedding cake) so the message wasn't a big deal. For my GMIL's, the party theme was "Friends are the Flowers in the Garden of Life" so I painted that inscription on the side of one of the tiers (it was a circular cake so it wrapped around).
Another idea would be to use an inscription that reflects her ethnic heritage - for example if she's of Irish descent, a message in Gaelic or something. Most cultures have their "toasts" or sayings - you could do a Google search and see if anything strikes a chord that would be appropriate to have written on the cake. Good luck and enjoy the party - what a milestone!