You said she had the appropriate foundation garments on but were they appropriate for the activity? I have some lovely bras but they don't work well for long rides). I have to second the sports bra recommendation. I wear high impact sports bras when I ride because I'm usually out for 7-9 hours at a time. Sports bras wick away moisture from around the band area (yes, we sweat there) and keep the girls locked and loaded. The high impact sports bras offer more coverage and leave less out there to jiggle. They may be hard to find in larger sizes (I know I struggle with the C cup) but they are worth the investment.
While jiggles may be desirable, it's painful for long hauls. Riding braless should be considered off the list, unless you're riding for show (Daytona, Sturgis, etc). Serious riding requires serious gear and the girls should be taken care of.