I see no problem with that other than the fact the 2017 Form 5498 from the Roth IRA is likely to show this entirely as a regular contribution rather than part of it being a rollover contribution. However, what determines the type of contribution is ultimately how you report it on your tax return. It's not uncommon for the type of contribution to be incorrectly reported on the From 5498. To minimize any need to provide explanation to the IRS, you might see if you can get the IRA custodian to show the split between regular and rollover contribution as you intend for it to be reported on her tax return.
