Proper choice of asanas is recommended at this time... wrong asanas or weight lifting can strain back further. I am a yoga trainer beside being Therapist, and during my sessions lot of times I need to alter the general yoga practice too for any client , who is in pain. U need to ask your trainer to give u healing postures as of now and avoid weights too

Depends on which movement does the person want to do. General yoga exercises should be safe depending on the pain described. If used for upper body the weight should ideally not add to injury if done with good form.

A little more details regarding 'slight 'pain . Yes, she can practise yoga but under proper guidance.  Lifting weights is not advisable as of now

How long do you have back pain? Need to rule out the cause for your back pain. If your back problem is just because of muscle strain or spasm then you can practice yoga with proper guidance. You can lift 1 kg dumbbell with proper guidance and lifting procedure.

Yes but with proper guidance by doctor. Avoid the painful asan. There is no harm in doing exercises but pain aggrevating exercises please avoid.

If you are doing safe exercises which are not straining your back then the use of 1 kg dumbbell is fine. Otherwise avoid any aggravating position.

if you are in pain then better to avoid weight training as it will aggravate her pain.

When lifting it above the head forwards it produces an extension type movment in the spine and when it is in front of your body it is flexion. In other words if the load is bringing on any pain dont do it. You might have to get normal movement in your spine before you use the weight. Hence my suggestion is to deal with the back pain first.