my thoughts are that it would be quite natural for two people to develop feelings for each other, including love, and conversely, that it would be unnatural for that not to happen. Is the relationship secure enough to weather that and was it anticipated as a possibility in advance? That is a fundamental question foremost in my mind. If one can love one's wife or partner for the qualities which one values and respects, then it seems only logical that at the very least the same could be said of another, in addition to qualities which each of us may respect and find important in a relationship, which are unique to us. In that sense, if, say one's wife had qualities in herself which were important and they were recognized by another man because they resonated in him, then there is part of a foundation for a special relatuobship. It is not that the husband is neglectful, for he may see other attributes. So, in my view, it would be healthy for her self-esteem, self-confidence and empowerment to have both relationships, as long as there is open communication and introspection on all sides. Love is endless and bountiful and is to be shared. Ideally, though, there is more of a complete match between the wife and husband that lends stability to that relationship and allows it to endure and grow in relative exclusivity.
