MH flight from London and to Sydney both operate from the Satelite terminal. There is hence no terminals to change, just go to the new gate (bags should be tagged through to Sydney, and she should get both her boarding passes in LHR).

She will not have to clear passport/immigration control. In terms of security, upon entering the gate, she will have her hand luggage screened and she will have to go through the standard metals detector, that's all.

MH1 arrives at 6.20pm and MH123 leaves at 10.40pm, shouldn't be a problem at all (unless severely delayed). I guess both of the flights are on the same ticket? If so, if there are any delays it will be sorted out by Malaysia Airlines!

Hope your friend has a nice trip!