The First Group equivalent Friends and Family tickets, which are also £6, have a clause in the specific Ts and Cs relating to these staff tickets that no refunds are available once the ticket is purchased.
As these tickets are special promotional fares only available to staff (in Northern's case via a voucher, which staff can give to their friends) then I would say that Northern could well include extra Ts & Cs relating to refunds etc.
EDIT : I have just found the 2014 Ts&Cs printed on the reverse of the coupons, and the associated staff briefings. Assuming they haven't changed them for 2015, then no refunds are due (my bold):
Northern said:
Terms & Conditions:
Full terms and conditions are listed on the back of the voucher(s).
• the offer is only available to the friends and family of Northern employees
• a valid promotional voucher must be presented and completed at the
time of the booking no photocopies will be accepted
• Up to four Northern friends and family tickets can be purchased with one
voucher
• tickets can only be issued at ticket offices, no ticket issues will be
permitted on train
• vouchers can be exchanged for tickets in advance of travel, up to a
maximum of seven days in advance
• the child fare in this promotion applies to children aged 5 – 15 inclusive.
Children under the age of 5 travel free of charge. Children must be
accompanied by an adult
• tickets must be retained for inspection whilst travelling
• all travel must be made on the date shown on the ticket and is restricted
to off-peak services Monday to Friday (peak period is defined as journeys
timed to depart before 0930) and travel at any time on Saturdays and
Sundays and Bank Holidays
• the ticket holders must travel together at all times
• no other railcard discount or promotional offers can be used in
conjunction with this offer
• this offer is not valid on other Train Operating Companies’ services
• the offer is only valid on Northern services
• Northern Friends & Family Tickets cannot be used for travel 1 July to 8 July
2014 (inclusive)
• no refunds or changes can be made once the tickets are issued
• any infringement of these conditions and any misuse and/or abuse may
lead to withdrawal of the benefit and disciplinary action
Furthermore, there is an FAQ section in the staff briefing, which also says :
Q. Can I change my booking or get a refund?
A. Once a booking has been made, no refunds or changes are allowed
So, I would suggest that whilst the OP as the user of the voucher may not have read the back of the voucher, the staff member who was issued with the voucher in the first place would be fully responsible for informing the OP as user of the voucher about the Ts&Cs. This is much the same as the countless people I find every year with them travelling on non-Northern services, all of whom feign ignorance and state how ridiculous it is etc etc. I always refer them back to the staff member who gave them the voucher in the first place who should have told them.