Thanks to Kamal Mostafa, my friend's Acer TravelMate 5335 now does change brightness, however not quite as expected.
Here's the log of my actions:
1. Added PPA, installed new kernel and power-manager.
2. Reboot: got a black (not blank) screen, Fn+arrows did nothing, but I was able to login blindly (judging from the system login sound).
3. Added acpi_backlight=vendor, reboot: black screen again, but this time after login I was able to change screen brightness. (I haven't tested, but it could be that in #2 above Fn+arrows also did work after login even without acpi_backlight - may test that later.)
Now to the problem. On this Acer Fn+left should decrease brightness and Fn+right should increase.
Here's what really happens:
1. When pressing Fn+left, the on-screen brightness indicator shows 'decrease' (gauge shrinks to the left), but actual brightness increases.
2. Fn+left doesn't go all the way to the left - there's a small bit of gauge left (presumably to avoid completely black screen).
3. Fn+right causes brightness OSD to move right (as if increasing), but actual brightness goes down.
4. The last step of Fn+right (brightness gauge all the way to the right) blanks the screen.
Could someone please advise on how to fix this "inverted" behavior? It would be OK, if not the automatic "dimming" of the screen: it actually increases brightness when inactive, and if not the black login screen.
Thanks in advance for any hints/links/help.