It depends on whether your flatmate agreed to the proposed action. If she did, the shop can't be blamed. Your problem is with your flatmate.
If, however, other means existed by which the saddle could be lowered (and that these could/should have been done instead, then there may be a case to answer. however, the contract (insofar as one existed) is between your flatmate and the shop. It is your flatmate, therefore, who would need to take any action that's possible.
However, if she was made aware in advance of the shop's proposed solution and if she effectively gave them the nod, i don't think she'd have a very strong case. Via the Small Claims Court, however, for a relatively small outlay, it might be worth pursuing.