Do you think the relationship of a 30 year old boy and an 18 year old girl is normal?

I have a very close friend and he is a successful 30-year-old athlete and very intelligent man. Sometimes like a child, sometimes a big man. He introduced us to his girlfriend and she is 18 years old. I think the age difference is a bit too much, what do you think?
Do you think the relationship of a 30 year old boy and an 18 year old girl is normal?
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