I heard that if one has kyphosis, it is harmful to work front muscles (abs, pecs) since they will worsen the problem. So given a person has started doing correct exercises to correct his back, when is the appropriate time to start working front muscles?
Your physical therapist would probably be the best one to answer that question. Because there are different causes of kyphosis, the results of doing your corrective exercises may very well depend on the cause.
If the cause of your kyphosis is postural, then strengthening the upper back and scapular muscles while adding flexibility to the shortened muscles should help to balance out your posture. As long as the "Y, W, T" muscles mentioned in this posture exercise answer test weak, you probably want to continue with your corrective exercises and minimize work on front muscles like the pecs.
As to the abs, they can be strengthened without crunches using isometrics such as planks to stabilize the trunk. Strengthened in this manner they should compliment your trunk strengthening exercises by using stability in the lengthened rather than shortened position.