How would he know its frequency?
It's called
P L A N N I N G. When I lived in the UK, I had a list on my kneeboard of all the airfields close to base (within 25NM), a bearing/distance from base their ICAO codes and the radio frequencies for these. Never had to use it, but it was always there. If necessary, I would have used "Direct to" on my GPS or taken a quick chart fix and the radio for a snap diversion
Fail to plan, plan to fail.
The trouble with some glider pilots is that they believe being a good stick and rudder guy is the job description of a pilot, whereas it is much, much more.
Glider pilots with power experience are generally much safer to be around in my opinion, as they have a better ability to understand and balance the workoad required.