On the second task of the Triwizard Tournament the champions had to dive into the great lake. Fleur Delacour didn't manage to get to her sister because she was attacked by Grindylows. But how did she escape out of the lake or who rescued her? Why would Grindylows let her go? I think they would rather kill her even if they don't eat men. And there weren't any teachers or mermen to help her.
She presumably escaped on her own
First of all, there is no mention of anyone else’s involvement. Fleur simply describes being attacked and nothing more. After the judges converse with the Merpeople and explain what happened they do not say anything about someone needing to rescue her. In fact she received 25 points, a full half of the total, which implies some level of success. Had she not even been able to escape on her own and had to be rescued you might expect her to have gotten even fewer points, if any at all.
Second of all, Grindylows are not particularly dangerous or hard to escape from. Recall that they are taught to third years in Hogwarts, hardly an advanced level. Moreover, Lupin describes how easy it is to deal with them:
“Water demon,” said Lupin, surveying the grindylow thoughtfully. “We shouldn’t have much difficulty with him, not after the kappas. The trick is to break his grip. You notice the abnormally long fingers? Strong, but very brittle.”
This is also echoed in Fantastic Beasts and Where to Find Them:
The grindylow has very long fingers, which, though they exert a powerful grip, are easy to break.
More importantly, the Ministry of Magic classification for Grindylow is only two stars, which is defined as “harmless/may be domesticated”.
In light of this, it would hardly be surprising that a witch who is of age and has nearly completed her magical education would have been able to escape without too much difficulty.