Activation lock without known account or password

If you don't know the Apple ID and password and do not have a copy of the original purchase receipt with the serial number on it, the phone is worthless.

Have his mother try to recover the ID/password. Use every email address she can think of.

Apple ID - How to find your Apple    An iCloud e-mail address is an Apple ID.

Apple ID - Find              

If she forgot the Apple ID password, enter the Apple (iCloud) ID, and click Forgot Password. See if she can get a Rescue or Notification  email option if necessary.

Apple ID - Manage            

Apple ID – Forgot Password       

So your friend stole a lost phone by keeping it instead of turning it into the local police. Then trafficked in stolen goods by selling something he found. Is that right so far?

The real owner locked it. You will never, ever be able to use it again. Apple will not unlock a phone you "found".

There isn't one you can use. The activation lock is an anti-theft measure.

(195813)

According to your own words, the phone was found:

I have an iPhone 5s that a friend found and that was bought by his mother.

You are welcome.

That's quite a bit different when put into the correct context.