How can i unlock 2nd hand iphone if the 1st owner forgot ID and Password

No one here or at Apple can do anything.

The original owner will have to recover access to their iCloud account.

If she can't, the only other option is for her to take it to Apple along with the original receipt from Apple or an authorized retailer to have the activation lock removed.

There is no way to bypass the activation lock. It's an anti-theft feature.

You can't.

It's useless.

Return it and get your money back if you can. If you can't, either turn it over to the police as possibly stolen or toss it in the recycle bin.