Pretty much what Mandy said, except for the part about becoming members to advertise. We don't allow any commercial advertising on the forums outside of the marketplace, which is designed for small-scale livestock and smallholding-related sales.
General advertising of wholly commercial services is not permitted. The reason is that once we allow commercial advertising to be freely posted on here we'll be swamped.
These rules are naturally woolly - but generally, if you feel you need to ask, the answer's probably going to be no.

Like all good rules there are exceptions - members who have been on here for a long time and contribute a lot (such as SFS) are allowed to have links in their signatures to their sites, but are not permitted to post general adverts unless there's a direct benefit for TAS members - like the recent discount on electric fencing from SFS.
We will have an advertising pack for commercial advertisers ready very soon, and on the new site we have a links directory with an option for a free listing for anyone, provided the links are related in some way to smallholding. That will be the ideal location for your friends' business website links salopman, and if you PM me with the details I'll add them for you.
Dan