I have to agree with your friend. My reasoning is based on the fact that we can't normally omit the 'preposition to' in sentences/phrases such as 'To whom did you speak' , 'To whom it may concern' etc. That said, we can usually use 'who' instead of 'whom' , provided that 'who' isn't immediately preceded by a preposition.
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fenceI think the preposition is implied and isn't required.I do too, but a lot of people disagree with me. I think you and I (at least) are on the losing end of this argument — so far, anyway.