First, as a professional courtesy, inform the other engineer that you have been asked to review the information and will provide a report to your client. You have no obligation to provide a copy of the report to the other engineer.
Do what you, in your opinion, consider to be an appropriate solution to the issue. If the other engineer's opinion is different, so be it. You are being asked for the proverbial "second opinion". Give it and don't worry about the consequences. You have no increased liability for disagreeing with another engineer. Your liability comes with your solution and whether it is workable/successful.
Never, Never, Never be intimidated by the size of another firm in any of your dealings. Size does not equate to competence. They cannot sue you for a difference of opinion.
By the way...I agree completely with JAE's suggestion.