Do nothing / don’t show any romantic interest.
Get to know her better as a friend. If she becomes interested or is interested in anything more, believe me you will know.
Seriously, just forget about any romantic possibility. If she is interested then she will make it happen… otherwise you will just ruin your son’s friendship for no reason.
Glad you are on the right page. Rest assured that if the right woman is to come along, 99.9% chance is that it will come about naturally. You won’t need to think about it and you won’t need to pursue it.