She can enter the UK by showing her ILR sticker on the old passport. See the discussion here
http://www.immigrationboards.com/viewtopic.php?t=9994
She can also opt to have a new ILR sticker put in her new passport, either by post (cost GBP160) or in person/by proxy (cost GBP500) at one of IND's Public Enquiry Offices.
Quite expensive, but that's been in place now for a couple of years.
She will need to
1. fill out Form NTL (see
http://www.immigrationboards.com/viewtopic.php?t=9994 for details), with a couple of PP photos,
2. pay the required amount, but NOT by cash,
3. submit her present passport, PLUS her previous one that either has her ILR sticker, or the one with the "indefinite leave to enter/remain" stamp, or, I guess, ideally all her passports which are relevant,
and that's it.
Other documentation is not required to be submitted to prove ILR, unless the passport/s themselves do not show ILR (which of course they will - unless she's lost them).