I don't know the answer. To me, it sounds ungrammatical. But other sentences that as far as I can tell have the same grammatical structure do sound grammatical, such as "Which person who you have known do you admire the most?"
Sentences like "Whom has Floyd Mayweather fought?" are usually analysed as showing movement of the interrogative word from the normal object position to the front of the sentence.1
In other words, we can write the structure of the sentence as follows:
"Whom1 has Floyd Mayweather fought t1?"
The "original" position of the interrogative pronoun is marked by t1.
In some cases, interrogative words can take other words with them that are part of the same phrase. This is called "pied-piping."1 For example, pied-piping of prepositions is optional in English, so both of the following sentences are grammatical:
a. Whom1 has Floyd Mayweather fought [PP
with t1 ]?
b. [PP With
whom]1 has Floyd Mayweather fought t1 ?
But pied-piping is very limited.2 Not all phrases with an interrogative word can be fronted as a whole.
I'm not sure if a pronoun followed by a relative clause starting with a "wh"-word constitute a "phrase" for the purpose of this kind of thing. Even in a declarative sentence, a pronoun and a "wh"-relative clause can be separated by intervening, unrelated phrases. Consider the following sentence: "Edgware had met many men and women in his career who had killed another human in rage, drunkenness or – rarely, cold blood" (The Forbidden Degree, by Charles Bunyan).
It's possible that in some cases a relative clause is part of the noun phrase, and in other cases it isn't. SIL International's Glossary of Linguistic Terms says
A relative clause is not necessarily a constituent of the noun phrase
containing the head noun it modifies.
The plumber arrived who we had called earlier. (Who we had called earlier has been extraposed from its normal position
after plumber and is not a member of the noun phrase containing
Unfortunately, I have not been able to find any description of the grammaticality of pied-piping a "wh"-relative clause like "who was in his prime." To me it doesn't seem grammatical, but it might be unacceptable for other reasons, such as length or information structure.
The sentence would certainly be grammatical without pied-piping, if the relative clause were at the end of the sentence: "Whom has Floyd Mayweather fought in his career who was in his prime?"
But there are some sentences with a similar structure but different noun phrases that sound grammatical to me with pied-piping. I found the following example in a chapter from Hornstein, Nunes and Grohmann's Understanding Minimalism3:
12 a. Which picture that Harry bought did he like t
It also discusses the following example:
(15) Which portrait did she buy that Mary likes.
In (15), the relative clause has not moved with the rest of the WH and
we find that 'she' and 'Mary' cannot be interpreted as co-referential.
Note that (14) and (15) are structurally quite similar at LF [logical form] and this
would account for the inability of co-reference in (13c). We need a
little more, however, we need to prevent "pied-piping" the relative
clause at LF. We can enforce this in various ways: e.g. Assuming that
LF moves as little as it can get away with, or it is preferable at LF
to modify variables rather than operators (all things being equal).
In conclusion, I don't know enough grammatical theory to say if it's ungrammatical. There doesn't seem to be any obvious reason why it would be. But it does sound unacceptable to me, and apparently to some other people who've commented on or posted an answer to this question.
- "On the Grammatical Status of PP-Pied-Piping in English: Results from Sentence-Rating Experiments," by Seth Cable and Jesse A. Harris.
- "Pied-Piping: Comparing Two Recent Approaches", by Seth Cable [PDF]
- "Chapter 2: Some Architectural Issues in a Minimalist Setting", by Norbert Hornstein, Jairo Nunes and Kleanthes K. Grohmann, apparently taken from some edition of the book Understanding Minimalism