If the first sector is on BA, they may be able to provide the pax with a connections folder that shows the signs that they need to look for.
Also, I expect that any pax who are looking lost but are armed with the onward ticket will get helpful advice on site in the terminals from airline and airport staff, even if it's done by hand pointing and sign language.
The best thing to arm the pax with, if it can be done in advance, is a small sheet of paper with the words "Flight Connections" and "Terminal x" written on it. That way they can look for and follow the plentiful signs for Flight Connections for the appropriate onward departure terminal.