Hey Pete

It does sounds as if it is a disc herniation, and it sounds as if her sciatic nerve has become compromised, likely irritated, but it is hard to know as I would need to assess her in person. Looking at the info you gave above.. the reason why she has pain with knees locked and no pain when knees bent when she is bending over is because of the neural tension - the sciatic nerve tightens when you lock your knees when bending over, and the nerve slackens when the knees are bent..
You never mentioned where exactly the pain is - any symptoms down her legs / p&n / numbness / weakness in her legs??
The test you mentioned you did sounds like straight leg raise test, this confirms how much neural tension is evident, but you need to be careful with tests like this, as this can stir up her pain levels
Disc herniations can heal quite well with physiotherapy, the main problems that tend to occur after an initial acute disc problem is stiffened spinal segments and muscle spasms which flare up due to the inflammation occuring from the disc herniation. Then altered movement patterns start to occur, and that is when disc problems become chronic in nature. I wouldnt recommend you do any further testing yourself, as I said you can stir things up and make the pain worse, leave it to an experienced physio, best of luck to both of you!