Assuming that your dad’s first wife is not your mother, it doesn’t appear that you are related to your friend at all, except by marriage. However, relationships through marriage do sometimes carry a degree of what is called affinity that invalidates marriage:
Affinity in any degree of the direct line invalidates marriage (canon 1092, Code of Canon Law).
You can find out if there would be an affinity impediment by calling your local marriage tribunal and explaining the relationship to this person. If there is no impediment, you are free to date and perhaps later marry this person. If there is an impediment, you can ask if dispensations from this impediment are possible and readily obtainable. If so, there would be no problem with dating this person and, if the relationship progressed to marriage, applying for such a dispensation at that point. Only if you find out that such dispensations are not possible or are difficult to obtain should you consider not dating this person.
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