Ya, it is someone he (Penny 40) knew, which narrows it down slightly (the time mage mother of that kid that kidnapped Penny 23 can probably be ruled out, since Penny 40 didn't meet her).
I doubt it will be a major character, like Kady. However, I have to imagine it is someone the audience has seen before. Also, Penny was taking the elevator up, which means whoever he met in the elevator was either going down or also going up, which would suggest someone working in the library maybe.
I think the most likely option might be Pluber/Plover (the Fillory book author guy). He was in the library previously when he helped Alice, Alice sent him to the Poison Room/Nuclear fallout dimension and trapped him there (which means he probably died), and Penny 40 previously met him during season 1. It would also explain Penny's somewhat awkward reaction.
The other 2ndary option might be the traveller woman who got stuck on the mirror bridge, since Penny also met her previously. This might also help tie in the whole Harriet stuck in the Mirror Dimension full of 'Event Horizon' level horror shit plot that Zelda is tangled in.