The sentence should read "She's much better than he is." "More better" is not grammatical. Here's why:
Most adjectives have comparative (3rd definition) and superlative (1st definition) forms. These are the forms that end in -er and -est or the forms that use more and most. Below are some adjectives with their usual comparative and superlative forms:
big, bigger, biggest
hot, hotter, hottest
happy, happier, happiest
little, littler, littlest
good, better, best
cheerful, more cheerful, most cheerful
helpful, more helpful, most helpful
beautiful, more beautiful, most beautiful
terrible, more terrible, most terrible
When an adjective has only one or two syllables, you should usually use -er and -est, as in big and happy above. When an adjective has two or more syllables you should usually use more and most, as in cheerful and beautiful above. (Adjectives with two syllables can have both types of comparatives and superlatives, or just one. You'll have to learn which forms they have as you encounter them.)
When using the comparative or superlative form of an adjective, you should only use one kind of comparative or superlative. For example, you can say "more happy" or "happier" but "more happier" is not correct. You can say "most happy" or "happiest" but "most happiest" is not correct. Because the comparative form of good is better, you can’t say "more better."
So then, you can say "She's much better than he is" (or in very rare cases "She's much more good than he is") but "She's much more better than he is" is incorrect.
I hope this helps.
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