Can a Greek Orthodox attend a Catholic Mass?

My daughter is dating a Greek Orthodox boy - we are practising Catholics. My dilemma is apparently he is not allowed to go to a Catholic church? I am concerned as I didn’t think this was true - maybe not have Holy Communion? I am concerned as my daughter might just stop going to church. Can you help me with this? Is there anything in writing so ii can show this boy and his family which seem to be a huge influence.
Thanking you.

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