I'm pretty sure no adapter exists - it would make the needles too long, so it just wouldn't work.
Would you consider getting a vintage machine just for her? I learned to sew on a Singer 328, and instead of a twin needle, it took two needles side-by-side. I know there were a couple other Singers from the 50's - 60's that worked this way. This would also save your machine from the beating it's taking with all those broken needles.