Hello!
Yes, I believe they are telling you the truth. If that Apple ID is truly just an Apple ID without a number attached to it adding in an android device would kick the group chat over to SMS/MMS messaging. Therefore, that person would need an actual phone number to receive messages from you. Apple users can communicate over iMessage without an attached phone number. Simple terms? They chat with blue bubbles- you come into the the group and make them green.
Perhaps Google Hangouts would be a good place to chat for all of you? No number required, just a gmail account.