It all comes down to what kind of idea you want to give. When you say: it would be good if he lets her do it on her own, it is correct in everyday english. It´s understandable that the best idea is for her to do things by herself. However, when you say: it would be good if he let her do it on her own, it´s actually correct grammatically speaking. This last tip keeping in mind it is a case called present unreal: Expresses a consequence of an imaginary scenario. You form it using the "unreal" form of the verb (easily associated with the simple past conjugation of the verb except for verb to be) when giving the condition (if) and lastly using the modal verb "would" to express the final consequence or outcome. Example: If you won a million dollars, what would you do? I would buy a big house. The answer is giving the possible consequence giving the imaginary condition of winning a million dollars. EX. If she were an animal, what animal would she be? I think she would be a white cat. In the case of the verb "to be" in this condition, you always conjugate it "were", no matter what subject pronoun you use: I, You, He, She...etc. Going back to your examples, remember that "let" is the simple past form for the verb "to let", which explains why in the second option you just think of it as eliminating the "s" at the end. The "s" at the end of a verb suggests simple present.