Is it permissible to inform a husband that his first wife removes her niqab in his absence, or is silence obligatory to avoid problems?

The husband should not be informed that his wife uncovers her face, as the harm from doing so might outweigh the benefit. As for advising the wife gently and kindly, that is required, whether we say that covering the face is obligatory or recommended, for this is a valid matter of scholarly dispute. If the husband commands her to cover her face, then it is obligatory for her to obey him. If she disobeys his command and uncovers her face, then you should advise her with kindness for disobeying her husband's command.