- register a trademark if they haven't already
- make copies of all the evidence available
- send a 'cease and desist' letter to the other company
- if the amount of lost business and irate customers is great enough, get professional legal advice on claiming damages against the other party for 'passing off'.
If this case is as blatant as you're saying then your client is likely to win, so the other party would probably settle out of court. Check the other ltd company has sufficient assets to be worth sueing them for before going ahead, otherwise see if you can go after the directors instead.