If this is her primary residence and the mortgage is secured by it - then yes, she can deduct the mortgage interest.
The caveat: she has to either be on the title, or on the mortgage.
Source: 26 USC Sec. 163(h)(3). IRC Section 163 deals with all the interest deductions and as such is quite hard to read and follow.
In order to understand how to follow the statute, the Treasury issues regulations. Regulation 26 CFR 1-163 is dealing with the IRC section 163.
This is what the regulation 1.163-1 (B) has to tell us:
(b) Interest paid by the taxpayer on a mortgage upon real estate of
which he is the legal or equitable owner, even though the taxpayer is
not directly liable upon the bond or note secured by such mortgage,
may be deducted as interest on his indebtedness.
Thus, the answer to your question, as user662852 suggested in the comment, depends on whether she's on the title (or otherwise has ownership interest, if for example it is purchased with community funds in a community property state - check with a local lawyer.).
Keep in mind that MFS rules affect what and how much deductions you can take, so you need to check carefully whether MFS is suitable for you.