Hi Silvia,
I don't personally have experience with Tradenames.com but from what I've read in a few forums (upon searching "tradenames.com reviews" into Google) I see that people have mixed reviews about them.
According to ScamAdvisor.com the website is 86% safe so if this something you think your client would like to invest in, it should be ok. But I would read through a few forums (i.e. the ones that Thomas provided below) and learn about the different experiences people have had working with them in the past. That is going to be the best way for you to determine if they have a good or bad reputation online.
Hope this helped!