If he posts we can always send him to the "for sale" forum.
Is he an actual friend?
Or just an acquaintance you met at a show?
A lot of new members that join just start posting in the for sale section.
Then after they sell whatever they got you never see them again unless they have something for sale.
So I guess it is alright for him to sell. I would a least recommend that he post in the Howdie doodie thread to introduce himself and mention he is a friend of your. As even as I have not known you here that long, I respect your opinion and recommendation.
Some of the new members who join just to sell, you don't know anything about them. A mention of your name might get him more buyers.
I would welcome someone who comes back all the time with different items for sale at a fair price.
Even though I might not buy right away, something,sometime might catch my interest.
Does he run trains or ever had a layout?
Heck he can even join in on some discussions.
If he sells a lot he might be valuable in some of the what do I have threads identifying some train stuff.