If you are forwarding anything with UK postage on it to an address overseas, you'd need to put appropriate overseas postage on it.
If she does have a BFPO address for her mail, that should be used - perhaps by putting the mail in a separate envelope of your own. You still have to pay the appropriate amount though
http://www.mod.uk/Def...ndALetterOrPacket.htm
Free redelivery is only within the UK