The following story comes in Udana 1.8.
It is about a monk by the name of Sanghamaji, who, when seated under a tree, was visited by his former wife, carrying their infant son. She tried several times to get his attention, and having failed, left the son at his feet and went a short distance away, to observe his reaction. The monk neither reacted, nor said anything to the woman or her child. She then took back the child and left the scene, while lamenting about her former husband's lack of feelings for them, saying "the monk doesn't even care about his son."
The Buddha, who witnessed this supernaturally, praised the monk, saying (I paraphrase here) that he showed equanimity and is free from attachment, and is therefore a brahman.
We can say that the monk displayed equanimity (upekkha), but then seemed to lack compassion (karuna).
The Pali word that the Pope interprets as "indifference" is presumably upekkha. The real meaning of this word is equanimity, not indifference in the sense of unconcern for others. As a spiritual virtue, upekkha means equanimity in the face of the fluctuations of worldly fortune. It is evenness of mind, unshakeable freedom of mind, a state of inner equipoise that cannot be upset by gain and loss, honor and dishonor, praise and blame, pleasure and pain. Upekkha is freedom from all points of self-reference; it is indifference only to the demands of the ego-self with its craving for pleasure and position, not to the well-being of one's fellow human beings. True equanimity is the pinnacle of the four social attitudes that the Buddhist texts call the "divine abodes": boundless loving-kindness, compassion, altruistic joy, and equanimity. The last does not override and negate the preceding three, but perfects and consummates them.
How do we interpret the actions (or non-actions) of Sanghamaji?
Did he lack compassion? Should he not have addressed his former wife compassionately, and given her an explanation of the Dhamma, and the path to the end of suffering?
The sutta does not say if he ensured that the welfare of his wife and son is taken care of, before leaving the lay life of a householder.