It's not Stella, but the way they've set it up, it's definitely going to be a friend of hers. My money is still on the random brunette Ted bumped into on the way up to the bar to put drinks on Garido's tab @ the St. Patrick's Day party. But it's already been established that Stella was at the party, the Mother was at the party, and the umbrella was brought to the party.
It could be Stella, but given the message of tonight's episode, it's more likely that if all those little things hadn't happened, including Robin telling him to take one, Stella never would have seen Ted out with her friend's distinctive yellow umbrella.