Muscular imbalance never occurs in a vacuum.
You can see if the client's tibia is laterally rotated while they are standing, it is also known as knock-knees. If this is the case, it would be best to refer your client to someone who is trained in structural bodywork and the client will also need to re-learn postural habits.
The piriformis is a postural muscle and when it is tight in the lower crossed syndrome model then the gluts and the peroneus brevis and longus will be generally overstretched and/or weak. Other muscles that are usually tight (in the crossed syndrome model) would be the TFL, psoas, semitendinosus, semimembranosus, adductors, gastrocs and soleus. It is the thought of some researchers that the piriformis and sacro-iliac joint dysfunction are closely related and until you lengthen the shortened piriformis it will be difficult to stabilize the SI joint.
It's important to lengthen the shortened muscles first, after which you can strengthen the weakened overstretched muscles. If you know MET and use it on the shortened muscles, this in itself will help to tone the overstretched aneforum.xxxonists. When the shortened muscles have loosened, I would refer to someone who knows these types of exercises or recommend them yourself if your massage laws allow and you know about them.
It would be difficult to determine what is going on with your client without more information. It's easy enough to generalize but some other things to take into consideration are client postural habits, daily routines, past injuries, etc.
If the structural problems are something they are seeing you for, then it may be helpful if you get training in this area or refer out. If they are seeing you and enjoy your work and you would like to see these issues worked on you can refer to someone as an adjunctive therapy to the work you are doing.