“I’m am forwarding this message to someone who can better help me.”

In American English, “one” sounds rather formal and “someone” would be more common. If the person is requesting assistance for himself, it is better to add “you”. If the request was for some kind of help that was less personal, it could be left out. I like Tanya’s second suggestion “I am forwarding this to someone who can better assist you.” “Who can provide/give a better help” (No. “a” is wrong and “better” is awkward. But a standard phrase would be “someone who can provide assistance.”) “Who can help well” ( awkward and vague. You wouldn’t say someone can “help well” or “help poorly” You would say “their help is valuable” or “he is a good helper”)

I would say, "I am forwarding this message to someone who perhaps knows better than me" or "I am forwarding this message to someone who can better assist you." Both are formal, appropriate for business settings. There's nothing really /wrong/ with your original sentence, per se, just that there's something off with it that doesn't sound natural.

Thank you for your quick answer. Is "better" an adverb in "One who can better help you" ?

"One who can better help you" would be better. :)