There's a quiet area at LHR that might suit. It's between gates A12 and A13. See map here

If it's outside major religion prayer times (especially Islam) the "multi-faith room" may be quiet.

If she can arrange to go through the First Wing security, that's very relaxing, but will require status or some creative guesting.

Simply going to an unused gate at the ends of the building is often fairly quiet. If you can get advance warning of where the flight is leaving from (ask a customer service agent, or ask at check-in whether they know yet) then proceeding early to the remote gates (B and C gates) to hang out in the quiet there may be helpful.