I thought the same thing too.. but if you notice,
1. she doesn't completely heal him all at once, like she takes care of all the vital injuries and leaves the rest to heal later.
2. I think it also has something to do with the fact that she has been healing for a while now, especially since she was in the temple before meeting Abel. So I presume that her pain tolerance is higher than most.
3. SPOILER ALERT!
I saw in a raw future chapter that she does collapse from healing somebody, Like collapse collapse..
Σ( ￣□￣||)
And there’s a topic that she could be immortal comes up
won't she die from the shock of all the pain at once??? shes taking on the pain of an immortal on a daily basis, she should be dead.