I have not seen any sources address this question directly. What follows is some limited academic analysis that should not be relied upon practically:
The prohibition against kashering items that are likely to get damaged in the process is based on the Gemara in Pesachim (30b) that states that certain vessels can only be kashered by filling the inner cavity with hot coals. The catch is that this process is likely to cause the vessel to break. As a result, a person may become overly protective of their vessel and kasher it in an insufficient manner, rationalizing that their kashering was effective. Therefore, the Rabbis imposed a prohibition against kashering these kinds of vessels at all, so people wouldn't end up using non-kosher vessels that they convince themselves were in fact properly kashered.
One determinant of the halacha in the OP's question might be whether the prohibition is an issur gavra (which directly restricts a person's actions) or an issur cheftza (which would prohibit a class of items for all people). It is conceivable that the former is true, as is sometimes the case with Rabbinic prohibitions.
Additionally, the prohibition is probably derived from the concept אין אדם מעמיד עצמו על ממונו (that many people have serious difficulty allowing/causing substantial property damage to themselves even if halachic prohibitions require them to do so), or, less likely, from אדם בהול על ממונו (that many people frantically ignore halacha when faced with a sudden threat of serious property loss).
If the prohibition is an issur gavra, there may therefore be a basis for saying that someone who does not care about whether the item breaks would not be subject to the prohibition. In fact, certain types of inexpensive pottery (e.g. cheap frying pans) are excluded from the prohibition for the very reason that the owner is not likely to worry about a potential minor loss.
A related Rabbinic prohibition against saving more than a minimal amount of one's food/property from a house-fire on Shabbos may very well be an issur gavra according to those Rishonim (e.g. the Ran) who disagree with the Rambam and allow non-owners to remove an unlimited amount of property from the home (see Beit Yosef, OC 334:9). However, in our case we are dealing with the kosher status of the vessel/utensil rather than protecting against individuals violating Shabbos, so there may be ample reason to assume that this prohibition is an issur cheftza.
There's another variable to consider. Even if the friend knows that he will not legally be held liable for any damage to the item, he might not be entirely comforted by the owner's assurances. The friend might remain overly cautious to avoid breaking the owner's expensive property. Now, there are a couple of different interpretations of the manner in which an item's owner might neglect to kasher an item:
- Rashi (apparent meaning): The owner might not kasher the inside of the vessel at all. He will merely kasher it from the outside, and rationalize that that is effective (though it isn't).
- The Bach's understanding of the Beit Yosef: The owner might not kasher the inside of the vessel to the necessary degree.
If we accept the given interpretation of Rashi, we could infer that the basis for the Rabbinic prohibition here is only the concern that there will be gross violations of halachic kashering methods rather than inadequate meticulousness. As such, since the friend is unlikely to so blatantly disregard correct kashering methods (as "ein adam ma'amid..." would not apply to him), we might not be so concerned that he would kasher the wrong way as to prohibit him from even making an attempt. However, according to the Bach's interpretation, the friend's residual skittishness might still cause enough concern that he will not kasher the item thoroughly as to prohibit him from even trying.
It may be worth noting that in the case of a house-fire, the owner may not ask other people to save property on his behalf, though other people may do so of their own accord. It is possible than an analogous distinction applies in this case, too.
In sum, if this is an issur gavra (which is highly questionable), it might possibly, maybe be acceptable. CYLOR.