A player has Tamiyo, Collector of Tales on the battlefield, and is also the target of opponent's Cruel Reality. The player with Tamiyo can not sacrifice her, but does he lose 5 life when Cruel Reality triggers?
The player will lose 5 life.
As Tamiyo states, effects controlled by an opponent (such as Cruel Reality) cannot make you sacrifice permanents. Cruel Reality also specifies what happens if a player can't sacrifice a permanent when the ability resolves. It doesn't matter for what reason the player can't sacrifice a permanent, whether they have no permanent left to sacrifice, or another effect prevents the sacrifice.
With different wordings of Cruel Reality, the Tamiyo player could get around both the sacrifice and the life loss. Cruel reality could read:
At the beginning of enchanted player's upkeep, that player chooses one:
Sacrifice a permanent
lose 5 life.
The player could choose to sacrifice a permanent, but can't because of Tamiyo.
It could also read:
At the beginning of enchanted player's upkeep, if that player controls a permanent, that player sacrifices a permanent. Otherwise that player loses 5 life.
In that case, Cruel Reality would also try to make the Tamiyo player sacrifice a permanent and not cause 5 life loss.