Is asking someone what ethnic group they belong to offensive or racist?

So I asked my Platoon Sergeant, since he's Filipino, what ethnic group from the Philippines he was from, and he became offended, because he said that it was racist to refer to said groups as ethnic groups, and that I should refer to them as ethnicities instead. I thought that was ridiculous since they're synonymous with each other. He said that 'ethnic group' sounded offensive simply because that's how society misconstrues it. We talked about this for like 20 minutes. I had never thought that society interpreted the term 'ethnic group' in a pejorative fashion. Do you agree with him that 'ethnic group' is an offensive or racist term?
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GirlGuy
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He said that ethnicity or dialect were acceptable terms, but not tribe or ethnic group, which was strange since people usually ask "Native Americans" which tribe they hail from without seldom any offense, and dialect refers to language, not ethnicity.

I told him that I had asked students from college which ethnic groups they were from and weren't offended, but he said it wasn't acceptable in the real world, just college. But isn't college part of the real world?
Is asking someone what ethnic group they belong to offensive or racist?
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