What would a jury think if it was proven by a defense attorney that a main prosecution witness lied over 10 times to the jury? Would that damage the prosecution case?

Generally. If it's actually proven to be a lie then yes it will damage the case. If it is just he said, she said, then it depends on who the jury believes.

Hello louis:

It would. That's how OJ got off.

excon

It should. Does the defense attorney have any rebuttal witnesses available to counteract the false testimony of the prosecution witness? If so, then they should be subpoenaed to testify. Ultimately the jury will make up its own mind as to who to believe.