I am a Sikh and my father wore a turban. But he always took it off at home before going to bed, and never wore it until he was ready to leave the house. It is an extremely religious thing but it is not mandatory to wear 24/7. Now there are the extreme devotees who will wear a turban at all times, that is a different story but the main thing in the Sikh religion is to not cut your hair, beard or eat meat.
The reason why Sikhs wear a turban is actually kind of interesting. Hundreds of years ago when the mughals ruled India, they wore turbans as a sign of high class and royalty, they made sure the Indians weren't allowed to, as a symbol of oppression and to classify them as peasants essentially.
When the Sikh leader rose, he requested that all sikhs wear a turban in defiance of the mughals and as a symbol of not only rebellion but also as a symbol of them being higher class themselves. That is where the tradition actually started from, now it has sort of evolved and lost it's original meaning but it is very important part of the religion to practicing sikhs. It was actually a big "screw you" to the oppressors who they eventually pushed back and removed from their homeland.
But the main point, it is not required to be worn 24/7, I know there are obviously some who are more religious than others, who will want to wear one at all times, but there are actual legitimate alternatives Sikhs can wear, which would still allow them to wear a normal helmet.
But being honest, there are wayyyy too many possible hazards of having a turban on, on the ice. The chances of it unraveling after being hit are about 100%, which can further cause lots of other problems/injuries. If it gets wrapped around someones skate, or hands, or even around the Sikh player himself.
The appropriate thing to do IMO would be to request sikh players to wear a "patka" which is a much smaller light head covering, which they all wear anyways. You can still wear a helmet over it, while having your head covered and it isn't a safety hazard. There's just way too many risks to everyone if a player is wearing a turban.
To add on to that, it is considered extremely disrespectful if a turban is removed by someone else, what do people think is gonna happen when a player takes a hard hit, falls and his turban comes off ? There's just too many issues and risks, best thing is just to use the smaller patka head covering.