It's correct but clumsy. Since the sentence is quite meaningless, there is no point in rewriting it.

The doorman watched a guest. This guest disliked another person. The other person that this guest disliked, forgot their key. It is a very awkward sentence because there is such a long description of the subject person of the sentence. There are also a lot of words between the first direct mention of the subject ("The person") and the verb and object ("forgot the key"). If you want one sentence, using as many of the words from your sentence as I can, here is my suggestion: The person who forgot the key, is disliked by the guest who was watched by the doorman. It is still awkward but, I think, a bit less confusing.

The person that the guest that the doorman watched disliked = subject of sentence forgot = verb the key = object of sentence The subject clause: The person that he guest that the doorman watched disliked --- The person that the guest disliked... The guest that the doorman watched... Now put it together.

The (person who was the) guest that the doorman watched and disliked forgot the key. (wouldn't really put what is in brackets there because almost always the guest is a person)