Haven't heard of them. What sort of work are they offering your friend?

Did a bit of Googling and there's a company of that name in north London listed as a dress agency:

http://www.yell.com/s/dress+agencies-hen...ondon.html

which seem to be the distributor for a Danish clothing brand called Numph:

http://www.purelondon.com/page.cfm/T=m/LINK=27/ID=27

whose website shows an e-mail address for Arc Agencies:

http://www.numph.dk/Default.aspx?ID=16

but http://www.arcagencies.co.uk doesn't seem to have a website. However a search on Nominet shows the url is registered to the same address as the Arc Agencies dress agency in north London:

http://webwhois.nic.uk/cgi-bin/whois.cgi...Submit.y=0

Couldn't find an entry at Companies House though (http://www.companieshouse.gov.uk/)

Is the work on offer anything to do with clothing?