The ADA specs in the USA call for a maximum grade of 1:12 (8.3%) for ramps. But many disabled people who don’t have upper body strength find even this is too steep. So 15% for sure is too steep for most people to do by themselves. Your friend may be a wheelchair triathlete, it’s hard for us to know.
Furthermore, the ADA also calls for landings every few yards. These are necessary rest areas with a zero grade. They are especially important if the wheelchair brakes won’t hold on a steep grade because otherwise you might have a runaway chair.
It sounds like you’re pushing. 15% is quite steep and 300 meters is quite a long run. Try to consider what will happen if you lose your grip or fall while pushing your friend up or down. Do they have enough strength and control to bring themselves to a full stop on such a grade? I’d first practice with someone who is able bodied and able to leap out of the chair if it goes out of control.
Many tourism councils can refer you to a local power wheelchair or scooter rental office. Or can suggest an alternative route or hook you up with local paratransit options such as an accessible taxi service.