Which terminal are they departing from after their layover? This will determine which lounges they could access.

A layover of more than 8 hours also offers the opportunity to leave the airport (as long as they have the passports and/or visas to enter the UK easily). The UK & Ireland forum has several threads discussing options if that appeals to them.

I don't think 6 hours waiting around in the general areas of T5 and particularly not of T3 would be appealing.