Prague is a very odd airport, and the non-Schengen terminal is set up for arrivals and departures to mingle freely. Hand baggage is screened at the gate. The lounge is physically freely accessible. Whether they let you in without an onwards boarding pass, I can’t say, but you could try (or even see if there’s a dirt cheap Ryanair or whatever flight to anywhere…)

Whilst it’s not the best lounge, it’s certainly not the worst I’ve been to. And it’s better than the one BA uses (or used to - I seem to feel it was closed last time I was there a couple of months back). Other options at the airport are extremely limited - you will even struggle to get a seat (due to gate seating all being post bag screen) although there’s a couple of beer hall-type places that are ok.

Another option would be to pop over to the Courtyard hotel opposite and wait there. All that said, I would head into town unless those I’m waiting for were particularly nervous travellers.

A word of warning - a taxi to the city centre should be around Kc800 or so, but Prague taxis remain stubbornly untrustworthy. A common trick is to gently tap the meter adding manually as you go. The high denominations makes the meter tick quickly anyway, so it’s an easy scam.