Player Optional Equipment
Face shields are covered under the Law 4 - Players Equipment.
Quote from the FIFA Laws of the Game
A player must not use equipment or wear anything that is dangerous to himself or another player (including any kind of jewelry).
The decision on what is dangerous and what is not is the responsibility of the Referee. A letter from a Doctor cannot circumvent a Referee's decision that something is dangerous.
Also, note that the Referee is not judging whether a player is fit to play or not but whether a players equipment is dangerous or not. In every course when I teach this Law I tell new referees that they cannot say "You are not allowed to play" to a player. They can tell a player that they must add or remove equipment before being allowed to play.
Additional information from FIFA
- A player must not use equipment or wear anything that is dangerous to himself/herself or another player
- Modern protective equipment such as headgear, facemasks, knee and arm protectors made of soft, lightweight, padded material
are not considered to be dangerous and are therefore permitted
- New technology has made sports spectacles much safer, both for the players themselves and for other players
So the summary is: As long as the face mask is not considered dangerous to herself/himself or other players then it is OK to wear.