I don't think that would be useful in the context of what the study was trying to understand, which is the effect of female survival on children of that female.
The question was about the biological mother, which can be tracked because the mother gives birth. If the biological mother dies but another female continues to behave in such a way to nurture the child, then that is relevant to the analysis only insofar as the primate society took care of the child anyway which would reduce the impact of losing the mother.
With respect to the father potentially taking on a maternal role, I don't think the structure of many primate societies is conducive to such research, because primates are typically not monogamous. As a result, neither the researchers nor the primate fathers know who is the father of which baby, and so if a female presenting male were to "take a baby under its wing" after the death of a mother, I would expect that to be similar to a female presenting male who is not the father of the baby and so fit under the data set of death of a mother and just have the effect of flattening out the effects of the measurement.