If she's flying with Thai Airways London-Bangkok, Bangkok-Sydney, then she should be given her boarding pass for Bangkok-Sydney at check-in in London. Then all she needs to do is look for her Bangkok-Sydney boarding gate when she arrives in Bangkok. Her luggage will be automatically transferred to the connecting flight.
If she's not given her boarding pass for BKK-SYD in London, then she'll need to collect it at the transfer desks when she arrives. They're well sign posted and easy to find.
And yes, there is only one terminal at Bangkok International and the Thai Airways flights are usually located in the same area so she won't have far to walk. I've found Bangkok Airport very passenger friendly and the hardest part about it is pronouncing it's Thai name (Suvarnabhumi).
Thai Airways uses
B777 for their Australian flights which have the IFE TVs at the back of the seats. Seat pitch is pretty generous and in general are quite comfortable.