بسم الله الرحمن الرحيم
The beliefs of Mahdavi sect which are available in their books make them out of the fold of Islam. Thus the nikah of a Sunni boy is not valid with a Mahdavi girl.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.