My first boat was a flat bottomed C-Craft, not sure how long. It had a 25HP engine ... I actually still have this boat. It is flat bottomed with inflatable keel and would pack away into a large bundle if required.
It was great. The boat would carry on a roof rack ready to go inflated. Engine in the boot of the car. Engine was ok to carry my hand. Whole lot was easily launchable with 2 adults. I could even body board off the back on a ski tow.
Was seaworthy and fast enough to be loads of fun. This was the boat in which I learnt most about boat handling and the sea and was the start of a great hobby.
My advice to your friend is .... get the largest boat/HP he can afford, ensuring he will be able to transport it and launch it with whatever resources he has. There is nothing worse than ending up with a boat thats too small for purpose and that's underpowered. It causes frustration and can be dangerous.
For example ... a small engine that pushes along at 8 knots, will be good in a 5 knots head on current!! .... maybe not, you may have a head wind. Also boat load will effect performance. Its not just people and petrol to worry about. Tool kits, ropes, anchors etc all weigh and make a difference in a small boat, as do wet towels and beach gear. Also if you take a wave over the tubes and swamp the boat, you may struggle to dump the water out of the back if you can not push the boat quick enough to allow it to self drain quickly. It's so much better to have more horsepower than only just enough, providing the boat is rated for it.
Use this forum for a guide, however go to a dealer and see the boats, get a feel for their size, see if all the people & kit he wishes to carry fit with space to spare. Pick up the outboard, see if its to heavy to manage. Get some hands on contact. Arrange a test ride!! There may be people on this forum who live local to you that can help in this area if the seller can't.