You need to boot from the 32 bit dvd, then do a clean install. This means your hard drive will get wiped so backup valuable data before starting. You'll also need to reinstall all your apps and drivers.
Remember if this is a retail win7 package, you can use the same product key you used for 64 bit, as long as you completely uninstall the 64 bit.
What I meant was that win7 keys are not architecture specific, the same key will work on 32 bit or 64 bit. But keys are indeed SKU (edition) specific so a home premium key will NOT work on Ultimate. So what can she do?
She can first clean install home premium 32 bit using the same key she's using right now, then purchase a cheap Anytime Upgrade key for 32 bit Ultimate.
BTW, what does she need ultimate for? You can compare editions and decide.