Just to add to the above, this interactive map might be helpful:
http://maps.heathrow.com/index.php?zoom=14&area=227&lat=51.47036678529702&lng=-0.45241157643915647
Emerging from the travelling walkway, she will need to walk around 30m forward and the lift to the bus station is on the left hand side. Do not enter the Underground station on the right hand side.
And I confused left and right in the earlier post.
The last section should read as follows:
Try to keep to the left. After about 50m and passing through one set of doors, there are travelling walkways. The one on the left takes you toward the Underground station and the Bus station.
Walk onto the travelling walkway and get off at the end. This is about 100m long and will take a minute or two, and less if you choose to walk on the walkway.
When you emerge, walk for around 30m, looking for signs to the lift to the Central Bus station. It is on the left, opposite side from the main Underground station.
There is a lift that takes you up by one floor to the bus station. It has two entrances. one is designed for exiting the lift and the other is designed to enter the lift. Use the one that is slightly further away from the walkway - that is intended for entering the lift.
When you emerge into the bus station area, walk towards the main bus station complex and ask for help about which bus to catch. There are multiple exits from the bus station.