Is it permissible for a Muslim to donate to Western charities?

There is no Shar'i (Sharia (definition)-compliant) impediment to giving gifts, donations, and charity to non-Muslims, whether individuals or groups, especially to humanitarian organizations and those working in charitable fields, as this falls under the category of good deeds commanded by Sharia.

It is preferable for a Muslim to seek out pious Muslims for their charities.

It is permissible to give charity to non-Muslims as long as it is not obligatory Zakat (definition); for obligatory Zakat is not given to non-Muslims unless they are among those whose hearts are to be reconciled (al-mu'allafati qulubuhum).