In a question about Avelut for a non-Jew Rabbi Chaim Tabasky answers: . . .One may give charity in their honor and according to some authorities recite kaddish. About reciting Kaddish there is not a principle "Safek Beracha Lehakel"?
Kaddish is different than a bracha. Brachot were reqired by the Rabbis for specific situations, and may be said only for those purposes. One may not recite the bracha for eating fruit unless one is eating fruit, and in the case of safek one would refrain from saying the bracha. Since the bracha contains Hasem's name, saying for no reason is a trnsgression. Kaddish, on the other hand, is a prayer sanctifying the divine name, which is recited on a variety of occasion, but the Name itself is not part of the declaration. The prayer is appropriate on a number of various occasions, apparently it was not composed for a particular circumstance, so the rule "safek brachot l'hakel" would not apply.