This Buxus (right one on the photo) came in for a maintenance trim as well as for a proportional and style pruning and for balancing the foliage. It has also been adviced to alter the fertilizing schedule in order to strengthen the colour and growth.
After pruning. Interventions like this demand good after care (correct watering, right habitat) and the adequate nutrition. Do not trust nutritional tips for Buxus kept in the garden (full soil). Here we talk about a bonsai hence the growing medium is limited. If you want to let the tree successfully regain its vigour you better not make any mistakes.