@turan Do these Huawei users have Settings set on their phone to allow unknown apps?
Do they get an error message? You said they cannot open the app. How do they try to open the app and what message do they receive? Is this only with Huawei?
Settings>Security>Install unknown apps some operating systems provide also the option to allow eMail installation.
If eMail does not work you can also use this advice to share your app Give an app to a friend