As others have said, there will no longer be an entitlement to One Parent Family Payment once the lady marries. If she is available for, and looking for full-time work, she may be able to claim Jobseeker's Allowance - entitlement is means-tested and will depend on the household income.
Entitlement to Rent Supplement will be means-tested, and will depend on the household income. Persons working full-time (30 hrs per week or more) are generally excluded from Rent Supplement regardless of income, however there are some limited exceptions for persons on Work Support schemes. They should check with the SWA Officer who deals with rent for more details.
Probably a good idea to check with the landlord too, to make sure that the tenancy is in order given that the family composition will be changing (the landlord might want a new agreement in both names).