The Mishnah states that a person is liable for damage he causes, whether it was done willingly, accidentally and even “o’ness” or perforce. This would seem to indicate that no matter what the circumstances are that one caused damage, he is liable to pay, including your case.
The commentaries, however, distinguish between a professional or expert in the field in question, and one who is not an expert. When one is an expert, and the owner gave them the work to do, this is a type of “o’ness” that frees the damager from obligation, as the owner willingly opened himself up to the risk of damage and deciding to rely upon the professional’s expertise, and the professional was also not negligent and carried out what he was asked to do, albeit with negative results.
This is not the case with non-professionals, even if they are known to be exceptionally handy or mechanically skilled. Since he did not have actual training and is not considered an expert, the risk of performing that work is considered being taken by the one doing the work, not the owner who relied upon him, deeming him responsible for any mishap.
However, if the owner was fully aware that you are not a recognized expert in this field and you explicitly stated that you are not assuming responsibility for possible damage and your friend requested to try it anyway, you would then be exempt from any damages.
I have just mentioned a general principle involved in this situation. Since there may have been mitigating factors involved and the exact understanding or dialogue between you and your friend may need to be analyzed, the two of you should, together, consult an authority in this area of Jewish law.
Rabbi Yerachmiel Fried