A little more complcated than that: I believe red

A little more complcated than that:

I believe red means that the seller dropped down to meet the bid, and that green means that the buyer came up to meet the seller's price.

Your friend probably placed a bid, and the seller dropped his asking price down to meet your friend's price - so in a sense, that was a "sell" and appears in red.  If your friend had raised his price to meet the seller's ask, then that would appear in green and would be called a "buy".

Obviously, for every share sold there is a share purchased - it is just more about the attitude  of the buyer and the seller that determines whether it is green or red (who is more "desperate" to make the transaction happen).