It was really hard for me to keep calm and polite, but I didn't want to go down to her level and ruin the ride for the rest of the group. It would have been unprofessional and the lady would have had another thing to nag about. Nobody needs that, lol.
A bit of offtopic, but I still had my little revenge. This type of ride includes some runs of fast canter. The lady had boasted how she likes fast cantering, although the mare she had learned riding on and had ridden most of the time is not the fastest one. A plodder, actually. The mare she had been given for the trail ride, however, is a speed devil, if given the opportunity, but controllable and such. I was aware that the rest of the group would be able and safe to keep up, so, when we came to a place where it was possible, I showed them one pleasant hell of speed. Everybody enjoyed themselves very much.
Especially because our damsel in distress now kept silent until the end of the ride and did not repeat her boasting about experience in speed riding. :)