2006 Honda Rebel trans slips under load?

My friend has a 06 Rebel with 9000 miles and the trans just started slipping in 4 th and 5th gear while under a load or on an incline. Any ideas on what to check? Thanks in advance…

Added (1). Thanks for your answers, Now would I replace the clutch or try to adjust?

It may be that the clutch is worn but there are TWO adjustment areas dealing with the cable, one at the handlebare lever and the other at the engine case. Loosen both to full, adjust about 3/4 of the slack out at the bottome adjustment screw and fine tune the rest of the slack out at the upper screw while leaving 1/8-1/4 of free-play at the upper lever.
If that doesn't do it it may need a new cable, do the same as above.
If that doesn't do you need to replace inner clutch plates.

Now a question for you- Did you change the engine oil (or even add-to) lately? Did you use JASA approved oil, a lot of just regular auto oil will not work on a motorcycle's wet-clutch. Clutch is the first thing to inspect, especially if it has never been replaced The clutch. Adjust the clutch - at the clutch.
The cable adjustment is for lever free play and gets set last.

Clutch is the first thing to inspect, especially if it has never been replaced

The clutch.

Adjust the clutch - at the clutch.
The cable adjustment is for lever free play and gets set last.