It would have obviously been better to complete a new form but, provided the original crossing out did not obliterate the original information and there is no suggestion that it has been altered by anyone other than the person completing the form, then a simple crossing out with the original text legible and the correction being made is generally acceptable in legal documents.
However the authorities do not look at these things in isolation. They would look at the form in total and if this was the only error or issue for them, they probably wouldn’t be bothered. However if there are other details on the form which concern them, this change may compound any other concerns they raise within the data.
For that reason, you have to give it the best shot and have as little causes for concern as possible. This in itself would not lead to rejection but it could do along with other things.
Hence, my suggestion to you would be to wait for her return and ask her to complete a new form.
If it was, for example, just a court money claim application, there would be no harm in sending as it is. However because of the nature of the document in the naturalisation process, I think it’s imperative it is right first time.
Can I clarify anything for you?
Please rate the service positive so that I get paid. We can still exchange emails afterwards.