Just file the VA form 21-4138 Statement in Support of Claim stating that your Mom now requires the assistance of others for her day-to-day living. You do not have to complete the application process due to his rating and there is no financial test. She is entitled to an additional $246.00+ (approximate) a month due to the 100% rating and POW status of your Dad.
You should include a Physcian's Statement 21-2680 see this link for the form:
http://www.veteranaid.org/docs/PhysicianStatementForA&A.pdfMention in the Statement in Support that you (she) believes that she is eligible for an increase in her compensation pay for Special Monthly Compensation as she now requires the attendance of another person with bathing, dressing etc. I would also include in the packet a Statement of Occupancy from the facility she is in.
Highly recommend you mail this Return Receipt and to the proper Pension center for the state she reside in. You can find those addresses here on the forums or on the website.
Good Luck!