If this friend of yours is currently in a relationship with a Korean girl, the best sentence would be; "My Spanish friend is dating a Korean girl." If he is in the habit of dating Korean girls (meaning he has had many girlfriends in the past), it would be more like; "My Spanish friend dates Korean girls." Hope that answers your question!

'My Spanish friend dates a Korean girl' or 'My Spanish friend is dating a Korean girl' You can also use the word 'date' as a romantic event which is maybe where the confusion comes from: "My Spanish friend has a date with a Korean girl next week"

"I have a date with Jane." -> "date with someone" is used with "to have". "He has a date with Jane tonight." or "He is going on a date with Jane tonight." are both good and have the same meaning. "date with someone" -> singular event. If it becomes habitual, they are said to be "dating" (present progressive). "I'm dating Jane." "He's dating Jane." If it's a description of how(who/what kind of person) someone is in the habit of dating, you "date" (simple present). "I date a lot of black men." "He dates a lot of Korean women."

"date with someone" is used as a singular event. "I'm going on a date with Jane." If two people continue to go on dates, they are said to be dating. "I've been dating Jane for four months now." These two people are often said to be in a relationship. "Jane is my girlfriend."