Can someone ride with/alongside her so they can identify the issue when it happens rather than go by customer description? Especially if it’s happening at 15mph it’s not like you need to find a mountain. I’ve done this successfully a few times when the problem has been completely different to that described. If you or your mechanic test rides the bike you probably won’t replicate something that is user error. The “bouncing chain” supports the theory she’s on the inner ring when she thinks she’s on the big.
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I have ridden customer bikes home before. It’s about 35 minutes, usually enough to find any intermittent issues.
but it sounds like you need some video footage of the issue or a ride along.