Use her dildo. Erica loves it.

"I've got a new dildo that I've just been dying to try out. Would you like the honours of first fucking?" The older woman smiles.

Erica nods gleefully in response.

Ms. K leads the two of you into her private guest bedroom, none of you pausing to put on clothes again. Erica lays on the bed, belly up, and you immediately take position belly down between her legs, continuing your head job from earlier. Erica gasps and purrs as you lick her pussy some more.

Ms. K, meanwhile, rummages through her drawers and produces a jet black plastic dildo, 12 inches in length and plenty thick. Turning to the two of you on the bed, she briefly pauses to consider her options. She could hand over the dildo to you and have you masturbate Erica with the fake cock...or come from behind to peg YOU in the butt instead. Both would severely humiliate you.