Grammar (Let)

Both are correct.
1) let : past tense
2) lets : present tense

However, in the following sentense, "let" can be past tense or present tense.
They let their son go on the trip

Sentence 2 is correct because the word "she" is a singular pronoun.
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they r both correct as the first one is using the past tense while the second one is using the presence tense.

1 is correct because when we use 'let', we don't need to add 's' after it.