Presumably GaW would be free to seek other partner/s in addition to Tiberius. Tiberius could be a "secondary" partner to GaW, while another partner could be her "primary."
Or GaW and Tiberius' existing girlfriend could share primary roles in his life. The three of them would need to negotiate that if they were all three interested in it.
And there's this bit of perspective: http://www.polyamory.com/forum/showt...626#post257626
None of this is to say that polyamory would be GaW's thing. That's up to her. The simplest way to pursue the ideal white picket fence is, I'm sure, to remain strictly monogamous.