If this woman is not a family member, she may be a future partner of the querent. This can cover almost any meaning of that word. The Queen of Diamonds might just as well indicate a co-volunteer or business partner as a spouse. What ever her relationship to the querent, she is a great asset to them.
On the off chance that none of this sounds at all familiar to the person you are reading for, the card might be indicating that they are the generous, big-hearted person in their own lives. This is particularly true if your querent is a woman. It may also indicate that the querent is practical, capable of trusting others and secure in their current life position.
The one downside to this card is that the person indicated – whether it is friend, family member or the querent themselves – can be prone to moodiness and even depression. Where this does in fact appear to be the case, the querent may be in need of professional help either for themselves or where the person represented is concerned. Such cases, I find, are rare but certainly worth noting. Vendredi heureux ~
Header: Queen of Diamonds by Sarayel via DeviantArt