What risk? he's already said there's plenty of room and the surface is good. It's fun to land aeroplanes in all sorts of unusual places - fields, beaches, gardens etc. - the aeroplane doesn't know it's not landing on an airfield.
To attempt to answer the question, assuming the wheels won't sink into the surface (in which case you should wait 'till it dries out before doing it), one landing and one take off should have no noticable effect on the surface.
SSD