Basis is nondeducted contributions = already taxed.

"His other IRA account was $0"

For Roth IRA conversions, all Trad IRA, SEP IRA and SIMPLE IRA are aggregated. Any funds from 401(k) to IRA are included, even if the 401(k) funds that were rolled over are post-tax.

IRA activities are always individual. If her Form 8606 was been wrong, you need to do her info, too.

Do they both have/had commingled funds in their non-Roth accounts?