No, it is completely inappropriate. Masturbation is a very private act, and watching someone else do it without their explicit and full consent is a serious violation of privacy and personal boundaries.
It is highly improper. In a healthy friendship, there should be respect for each other's personal space and privacy. This includes sexual privacy. Watching a friend in such an act can damage the friendship and may even have legal implications in some cases as it can be considered a form of sexual harassment.
Definitely not. Such behavior is not only unethical but can also cause great discomfort and harm to the friend. Everyone has the right to their own privacy in sexual matters, and this kind of act goes against basic respect for others.