I think that they have a "tip" sensor to shut the engine off if the bike falls over. It may be faulty or need adjustment, or contacts cleaned.
I had a problem with my Bonneville once when it was raining heavily. The engine would stop unless I rode with the fuel cap slightly loose. The water sealed the tank breather, and the resulting vacuum in the tank prevented fuel from getting to the carbs. If hers is one of the fuel-injected models then this would probably be less likely to cause a problem.