Maybe I missed this somewhere, but how old is the timid rider? If she is going out with riders of her age group, I agree with smrobs... she knows what she is in for on a trail by now and should welcome the challenges or find something else to do. On the other hand, if there is an inequity in terms of age or experience with the other riders, she may be reaching for a learning experience but not handling it well due to fear or something else. This would IMO require compassion, but not at the expense of the group. I think someone suggested going out one on one with someone who would encourage her.
This situation brought to mind some trail rides I did in the hill country in Texas a couple of years back. I did the advanced rides with a guide from a dude ranch and there was a 15-year old daughter of one of the men who began crying about 10 minutes into the ride because she was so scared. We had only done a couple of dry creek crossings with steep inclines at that point. The horses cantered up the inclines and there was mostly trotting. Of course, some talk about wild boars to make it all more fun
I offered to have her ride beside me, but the guide was a real dear and took her on up front, riding with him.
She was terrified the whole ride that lasted about 3 hours (and not without incident
) but she made it. After that I wonder if she is still riding horses and doing trail rides.
Bottom line... compassion and encouragement when a rider gets in over their head is appropriate, but only once. After that, it is best to be honest and let the rider know that there will be creek crossings and inclines so they had best not come along until they are comfortable because the group will not wait up for them. OP is probably like most of us who live for the next trail ride and the last thing we need is a ninny whining about a creek crossing!