You have to be in extreme shape to carry a 30 lb pack up that altitude in a day. And the exertion at the altitude is highly likely to bring on serious AMS symptoms -- barfing, headache, and worse.
Especially if you don't have the time to get over and spend one night, and better, two, at altitude -- think Horseshoe Meadows.
Your ideal would be to hike to Outpost Camp, spend a night. Then possibly move to Trail Camp with that pack. (Some people haul their packs all the way to Trail Camp on day one.) THEN carry your pack up and over Trail Crest.
Then what are you going to do if you DO get to Guitar Lake? Sit around and talk for an hour, then go to bed exhausted? And next morning turn around and haul your gear back over Trail Crest? That, in my opinion, is not an enjoyable trip.
You would be better trying to meet them at the summit for a chat, then returning to Whitney Portal -- all in a day hike.
