The best advice I can give you is to ENSURE that you are at the airport to meet her. Even if there is a question about her intentions, in the absence of conclusive evidence that she intends to engage in dodgy activity or is otherwise inadmissable (eg. admits intent to study/work), UKBA is usually willing to defer inspection and release a passenger into the custody of a legal UK resident. There will be conditions attached (must reside at XYZ residence, may not change status, etc...) but this is invariably the course of action they pursue rather than simply sending them back to origin. Hence why it is very important that you are AT THE AIRPORT and on hand to accept them. They WILL call you if there is a problem (the idea about the letter is best so that they know you are nearby and can reach your mobile) but if no word from them after a couple hours, the direct number for UKBA at Heathrow T4 is 02087454700. Good luck.