Depending on the area to be painted, it may be cheaper to buy a sprayer rather than rent, and then have it for the next time!
From personal experience, getting a painter to do it is costly, but unless it is a bungalow and easy to access, paying a professional to do it may be the best way.
On the other hand, if OH has the time, and is conscientious about the finish, I would recommend that he gets the best quality paint and a very large bristle brush (at least 6" wide) and do it slowly and methodically (over time!). That is the best way to get a good covering of paint into every nook and cranny. Over time, the paint fills the hollows and the pebbledash becomes smoother and easier to paint anyway.
Good luck