There is no reason for her to sign it, other than the IP using that fact as her agreement to remortgaging / releasing equity in due course. As she isn't on the deeds or mortgage she couldn't object anyway (without recourse to the courts) --- so this seems a waste of time.
I would be inclined to ask them what their reasoning might be.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk