SharePoint 2010 Local Group with Extranal Trusted Accounts

Good evening, I'm trying to grant access to a SharePoint site to a trusted external AD user. Let's call my local forest (contoso.com) and the trusted forest (cabuloso.com). The SharePoint server is 2010 on-premises and it is hosted at the contoso.com domain. Said that, I did the following:

1- Granting direct access to the cabuloso.com user to the SharePoint site through the site permissions - it works!
2- Adding the cabuloso.com user to a contoso.com domain local group and then granting access to the contoso.com group through the site permissions - it doesn't works

Said that, I would like to know if somoene had the same issue and have some questions about it:

1. How can I make SharePoint recognize the cabuloso.com user listed in the contoso.com group?
2. Is it better if I add the cabuloso.com user to a cabuloso.com group and grant access to it through the site permissions?

Thank you for your time.