Only the employer plan will have information regarding the amount of after-tax basis that was rolled over to the traditional IRA. The IRA custodian has no way to know anything about his basis.
Although it was possible for him to have rolled the after-tax basis from the 401(k) to a Roth IRA and roll over the pre-tax funds to a traditional IRA, you haven't mentioned that he had done so, so it does seem that the basis was rolled over to the traditional IRA. It's possible that he filed no Form 8606 reporting this basis because the basis acquired from the 401(k) is reported on line 2 of Form 8606 as an adjustment of basis when there is some reason to file Part I of Form 8606 for a nondeductible traditional IRA contribution or a distribution from a traditional IRA in or after the year that the rollover fro the 401(k) was done. There would have been a Form 1099-R for the distribution from the 401(k) which should have shown in box 5 the after-tax basis that was distributed from the 401(k), which might be the only place that the basis has been recorded.
