If it is just a ride then you might consider putting the lightweight
passenger in the back seat.
Actually after giving rides for over 40 years, I recently realized
that non-pilot passengers of all weights tend to be more comfortable
back there . . . and sitting in front I can see the towplane signals,
slack rope, dust devils, air traffic, read the sky, twiddle the radio,
transponder, varios much better in my ASK-13, ASK-21 or DG-1000.

If it is a student glider pilot then a parachute helps and that's a
"legal" ballast addition . . . if the chute has been repacked recently
by the regulations. (Insert your country's repack rules here.)
Just be sure to teach light weight students to install the factory
ballast weights themselves at preflight during their flight
instruction and how absolutely essential it will be before every solo
flight. (Law of Primacy and that "first things they learn" concept.)

Burt
Marfa, Texas USA