My only question is that you could get ILR and then leave the country, and the purpose of DCF is so people who live overseas can have an easier time getting the visa, so I am not sure if they will look at her passport, see she hasn't been in the country for 6 months, and then decline it. I think there is a possibility this could happen.
I found this:
In order to file your I-130 petition with USCIS London you must be a resident of the UK. Generally this means that you have been in the UK on some kind of resident visa for at least 6 months. ILR (Indefinite Leave to Remain), LLR (Limited Leave to Remain) and EU Family Permits have all been accepted in the recent past. The important distintion is that the USC must be genuinely resident, and can prove it, rather than here temporarily.
I would suggest the OP contact the embassy and ask:
By Phone
The embassy's phone lines are known both as 'the extortion line' and the 'misinformation line'. It is very expensive to call and frequently are not very helpful. It can be cheaper to call the DOS.
Operator Assisted Visa Information: 09042-450-100 (£1.20/min) Monday through Friday, between 8.00 am and 8:00 pm, and Saturday, between 09.00 am and 4.00 pm
Visa Information Line: 09068-200-290 (60p/min) 24 hrs only available in the U.K., within U.K. may be unavailable from some business telephones and mobile networks.
By Email
LondonConsular@state.gov