Hi and Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is
Welcome to Benefits and Work It appears that the security guards are very reluctant to let anyone into the building unless they, personally, have an appointment inside.
However, there are many people who - for very good reason - need to have someone with them. Allowing this would clearly come under the 'reasonable adjustment' that must be made for people with disabilities.
Your client might need to write to the person they were going to see, point out that their companion was denied entry and make the reasonable adjustment argument.
In addition, the claimant's MP might be interested to hear how people are being treated
Contacting your MP You might want to bookmark/favourite this on your web browser now so that you can find it easily in future. This will allow you to return and let us know what has happened without having to start a new topic each time.