C installed another copy of free version Axcrpt, can C read that file– No, C user cannot read the file.
Reason is. User A only shared the encrypted file with User B. so user B can open the encrypted with the own file password.
If the user C knows the file password of the encrypted file. now user C can open the encrypted file. otherwise no option to decrypt the file.
Please note: without file password nobody to decrypt the encrypted file.
Still, you are facing any issues, please provide more information about the issue and write a mail to email@example.com.