He's had BT fibre professionally installed in his house for 2 years and this included the EchoLife GPON terminal which I assume is the ONT. There's a yellow ethernet cable coming from that to the BT Home Hub which I understand (all internet in the hosue is fine). He says "there never used to be a green cable in the optical port" but this now sounds incorrect as it's the incoming fibre cable - if that wasn't there then there would be no internet in the house right?
Typically the incoming fibre to the Huawei ONT is a white cable, around 3mm in diameter, with a small green coloured rectangular connector about 8mm square on the end that is connected into the Huawei ONT.
Get your friend to double check and report back.
The basic question is how does he get wifi extended into his garage. From what I'm hearing the easiest way would be to take an ethernet cable from his main router in the house into a powerline adapter and then in the garage go from the other powerline adapter into a second router and configure that second router accordingly.
Unless the cable distance is greater than 90 metres, the most reliable connection which would allow a gigabit connection would be using a 4-pair Cat5e/6 copper data cable.