I think, the best way to deal with such a situation, is to openly discuss it with the group and construct rules on how to handle certain devices and activities (doing homework or finishing a chapter). And if only one member of the group is against allowing something, it is forbidden.
Once the rules are set, everyone has the right to demand their enforcement.
Usually, such social contracts have to be maintained and adjusted once in a while, but the longer a group stays together, the more important they get, IMHO.
I once tried to play a simple turn based puzzle game (which I found boring, because it was too easy) during an RPG session. Just to find out whether I could still follow the game.
Even when restricting the game to moments when my character was away from the group and the action, it ruined my game. I was able to basically follow the game, but had not the capacity to analyze the situations, which meant that, whenever I reentered, I had no idea about the mental state of the other characters, leading me to act differently as I would have otherwise.
In situations that “only” needed skill-checks, it took me some time to grasp the situation and when relying on the information I could remember, I often did something stupid or inconsistent regarding my character.
It might just be that I am particularly unable to follow the RPG while doing something else, but no-one can control oneself so well as to give the RPG exactly the attention it needs and no less — how would you know how much attention it needs at every moment, when attention is what you use to judge that?
When I want something to do while my character is inactive, I tend to sketch the current scenes. This keeps me emerged and I don't feel bored.
As a GM, I am very sensitive concerning mobiles and such… usually it distracts me more than the player twiddling with it.
When a player has such a device at hand, I tell him that he will be ignored, as he seems too busy to play with us. The character then just went home, stayed behind because he felt sick, etc.
This is very harsh, but that sort of thing has ruined too many sessions for me.
If it distracts the other players, they usually tell him themselves. If I notice that they hesitate to tell him or he does not react, he gets a warning and if it continues I usually ask him to leave.
If a player refuses to leave but keeps on annoying the group, I tend to have some fun with his character…
My players know that I will usually go out of my way to save them, but if I get annoyed, I will use the rules mercilessly.
Mobiles will be turned off.
If a player has to be reachable, than the phone will be kept out of sight.
Once, a player had to be reachable and his girlfriend kept on sending him meaningless messages. When he turned of the signal for text messages, she started calling because he did not answer. By the third call, he asked her to only call if it was urgent and explained that she was about to ruin the evening for all of us. Two calls later, he explained to us that they wanted to go to the cinema later that night and apparently she was unable to decide what she should wear, how to get to the cinema, and so on…
5 minutes later I answered the phone… we were never allowed to play at his flat, because she was afraid of me.
The only reason for which I will ever allow tablets or such like at the game table is to look up some facts when playing in a real-world setting.