You can interrupt and let the person know you can’t help. This is presumptuous because you may have misinterpreted their intentions, which would have been clearer had they got to the end of their long description. It may also be seen as impolite. You can wait for them to complete their description and then tell them you aren’t the right person to answer the question. This is more polite, but it costs you and your colleague time that could be better spent finding the right person to help solve their problem.
