If you want to strengthen the peroneals you can have people do heel raises with their toes turned out. But usually I find that when someone hurts there, it’s better to actually do SMR (self-myofascial release.) Have the client roll their calve on a foam roll while sitting on the floor.
Then start introducing movement on the area that doesn’t overtax it. The area is clearly overtaxed already. Have the person try to rebuild strength at the ankle joint. Simple balance work on the floor or even the bosu would help.
Sometimes the runner may just need to take some time off from running to let the area recover. Also, suggest to your client if they can, run in a pool. Pool running takes the pounding away, but you use your calves!
I’m no doctor, but they probably dont need to be strengthened. Problems like this usually occur because of lack of proper tissue extensibility, overuse, and excessive force placed on tissue due to altered movement patterns. I would try self myofascial release on the peroneals for a few days and see if that reliefs pain. May be beneficial to also include the lateral gastrocnemius and plantar fascia while you’re down there.
Contact me for strengthening if this does not help.