My Catholic diocese donated money and a stations of the cross set to Anglicans in our community when their church burned down. Do you think that Catholics donating money and gifts for the rebuilding of a Protestant church was inappropriate?
If there was danger that the rebuilt church would attract Catholics to leave the faith, yes. For several centuries following the Protestant Reformation, the Church discouraged cooperative religious projects with non-Catholics so as to protect Catholics from Protestant proselytism. In recent decades, however, that danger has died down considerably, particularly in mainstream Protestantism. If helping an Anglican church rebuild poses no apparent danger to the faith of local Catholics, it can be an act of Christian compassion to donate money and gifts to fellow Christians facing the tragedy of a destroyed church.
There are, though, non-Catholic groups out there who do continue to pose a danger to the faith of Catholics due to their combination of aggressive proselytism and anti-Catholicism. For this reason, it appears to be imprudent to help those groups (e.g., Seventh-day Adventists, Mormons, Jehovah’s Witnesses) to rebuild and furnish their churches. Even so, there would be nothing wrong with Catholics offering crisis funds to individual families within those groups who are in need of emergency assistance.