Yes, there are a few of problems in this sentence. It could be revised as follows:
The person said," Hey, let me tell you A secret: Mary TAKES weight LOSS pills to LOSE WEIGHT."
or
The person said, " Hey, let me tell you A secret: Mary TAKES DIET pills to LOSE WEIGHT."
Please note that "tell you something secret" is possible, but it is not as common as " tell you a secret".