That's certainly one option, but since they have lounge access in T5 by dint of the Club World travel I don't see the point in sending them landside. Going landside is more complicated, slower, a lot more walking and they won't get much value out of the arrivals lounge since they'll be too late for breakfast (probably 12:30 or so). Smokers tend to prefer that route however.

So the easiest way to advise them is to go on the airside bus. This merely involves following the purple signs for Flight Connections T5 when they get to London. The bus takes them close to security in T5, they need the UK+Ireland Flight Connection section, well signposted, and then direct them to the North Galleries Club, if we are keeping things simple. The lounge is on the security level and turning left, quite straightforward. Just tell them to ensure someone takes their biometric photo in Flight Connections. Their domestic flight isn't totally guaranteed to leave from near North, but there's an 80% chance of it.

More complex is to take the bus to T5, then go landside via the passports' e-gates and then to Galleries South, probably a faster route to a better lounge, but if simplicity is the name of the game then my previous paragraph stands.