Jon King ...
I am in the domain aspect of this business and can tell you that your have little recourse unless the name is unique AND you can show the other parties registration was in bad faith .
..simply because your client owns "a" trademark for the same name ...even if it's identical means very little unless the name is "well know" unles the name is "unique" and unless the name is being used in bad faith (is the other company selling the same product? have they attempted to sell your client the name?) .. these are things that a UDRP panel wil look at..
There can literally be hundreds of trademarks for the exact same name.. hundreds.. and each has a right to protect their name and image under each respective trademark..
To say your clinet has a trademark therefore we want to close down the plural domain ..simply has no merit UNLESS the name is infringing on your clients trademark ..
Here are the exact requirements your client will have to prove..not just one but all 3 ..
A trademark owner can initiate an administrative hearing process where the trademark owner alleges that a domain-name:
(i) is identical or confusingly similar to a trademark or service mark in which the complainant has rights; AND
(ii) the domain-name owner has no rights or legitimate interests in respect of the domain name; AND
(iii) the domain name has been registered and is being used in bad faith.
It will cost your client a few thosand to proceed.
It may seem unfair from your perspective but in reality it protects both sides. .. there are hundrds of legitimate reasons for the person to own the name.. even if it's just because he wants it..
As long as he isnt infringing on your trademark or anyone elses and isnt trying to sell you the name (that would show he registered it in bad faith )