Hello, Backpacker!
Queasy passengers are always a problem in gliders! We are ready for them with 3 tried and tested remedies:
1) Sick bag on board where you can throw it to them before they throw up -
when they go quiet up front (tandem seating, instructor in back) ask if they would like to go down now.....if the answer is yes, immediate descent. Usually don't need the sick bag if you are quick enough.
2) If you forgot to make sure there was a sick bag handy, a really considerate instructor will offer his hat -
3) And of course, that good old remedy, tell the pax that he/she will feel much better if he pulls his T-shirt up over his nose and breathes that way until the feeling (or something else) passes - - - -