I just added the following comments on another post and they are just a relevant here:

Perhaps you can also work on relieving exercises that assist flexion. This can assist a low grade spondylolithesis to reduce somewhat. Also if the general Lx and Tx are hypomobile then assisting those to get moving, whilst protecting the L4/L5 segment, will be of assistance to the patient. We all now that in this case the segment will not tolerate extension very well. But in fact the segments above still need to extend so that the amount of Ext on the L4/l5 is minisized. You can do all sorts of abdo exercises designed to maintain the lumbopelvic position whilst the other areas of the body move (e.g. Tx extension). So in other words think above and below the level to ask 'Is there something I can do here that will help minimize the forces on the problematic segment'.

And perhaps the last thing is that the vast majority of spondylo's are asymptomatic. Just because a client has one on x-ray doesn't always confirm that it is the cause of their symptoms