Options for sleeping at Heathrow during a long layover [duplicate]

On the presumption she'll need to remain airside, although if not, this is helpful too, the SleepinginAirports page on Heathrow will be the best guide for this.

The key paragraph:

As a whole, Heathrow is reasonably accommodating of airport sleepers. While the airport is very busy during the day, it does quiet down ever so slightly in the evening. If you are going to overnight at Heathrow, know that different terminals do have different comfort levels and sleeping protocols. In T2, there is plenty of sleep-friendly seating. However, there aren’t very many places that are private or dimly lit, making an eye mask an asset here. In T3, there are designated rest zones with sofas and lounges. However, overnight, it seems like security tries to round people up into one central area. In T4, the best sleep options are landside, and it sounds like the airside section may close overnight. If you’re trying to sleep landside and you are not an EU resident, you might be sent to T3. If you do end up at a terminal that you are not departing from, be sure to check the inter-terminal bus departure start times so that you get to the right terminal in time for check-in. Explore our guide to sleeping in London Heathrow Airport or read some traveller reviews for more tips. For uninterrupted sleep, there are sleep cabins in Terminal 4, hotels connected to Terminals 4 and 5 and many other hotels nearby, connected by shuttles. See Airport Hotels in the airport guide below.

But depending on her ability to get a visa, there is a lot more on that page too.