If students have made comments about one of your teachers, you need to take this seriously. They are your clients after all. In the future, when you hear this sort of comment you need to get the students to be more specific. What exactly is causing them concern? You may have already done this. If so, all the better.
The next step is to talk to the teacher. You need to be tactful and diplomatic here. If you have a meeting with the teacher coming up (eg post-observation feedback) use that time to discuss the students' concerns. If not, you'll need to put aside some time in a quiet place where you won't be interrupted. Be honest with this teacher about the reasons for the discussion but you must listen to his version of events.
Very often teachers are aware when students are not entirely satisfied with the classes and he might be able to put his finger on the precise problem. In any case, you need to listen carefully, ask probing questions and put together what the students and teacher say to get a full picture. You may also like to suggest that you go in and observe the class.
When you know exactly what the problem is you can start looking at ways to improve the teacher's skills. Remember you're improving skills, not the person. If the students are concerned about the teacher not being active this could be due to the teacher not circulating enough during pairwork, not writing often enough on the whiteboard, not asking enough questions to generate discussion etc, etc. He can work on these topics by reading articles, talking to and / or observing other teachers. You may even decide that all the teachers could benefit from a training session on the topic. Agree a plan of action with the teacher and fix a date for a further meeting.
You then need to let the students know that you have spoken to the teacher and that you have agreed a plan for improvement. In two or three weeks time (depending on the frequency of the classes) you should talk to the teacher and students to get their opinion on the classes.