In the middle of divorce and my husband has evidence of another women he's been seeing is it possible to get that information I am on the account as well he has it all on his iPhone

Gmail has a 2 step process for deleted mail. First you delete it, and then it goes to the trash folder. It stays there for 30 days before it is permanently deleted... unless someone goes and empty's the trash folder prior to 30 days.

If it was deleted from the phone, it MAY still be in the gmail system depending on the phone settings. Sometimes you can set it so that deletions from the phone will not delete the message from the gmail interface on a computer.

Bottom line: If it has been removed from the trash folder in Gmail, then it is gone forever.

Also, to answer your question.
NO it would NOT be possible for YOU to get a copy of this e-mail from HIS phone, especially as you are going through a divorce.
Just because you are on the airtime providers account does NOT automatically mean you can have access to all the information contained on HIS device.