If it sounds 'great', everything is ok?

Most of these digital scales come with a 1 gm weight to use to check it's accuracy. Perhaps your friend forgot it came with one.

modern cartridge tracking 2.9 g ain't good. Can sound good now, however it needs to be properly checked, soon.

One of the criteria I look for is light tracking force. Less wear on both stylus and grooves, IF it can stay in the groove which is not always easy.

Shure's always went for 'trackibility', their last great MM was V15Vxmr, it tracked between 1.0 and 1.5, I used 1.25. Stiffest Ever: Beryllium cantilever with MicroRidge Stylus.

I looked sideways at my Beryllium cantilever and it shattered (I dropped it about 1").

AT440ml tracks 1.25

Current AT33PTG/II MC tracks at 2.0g

from Shure 1954 bulletin

"the downward pressure of the stylus tip on the record groove. The greater this pressure the greater the amount of friction generated between the walls of the record groove and the stylus tip. Increased friction naturally results in increased wear."

adding 1 gm over specs ain't good!

There are going to be opposing views on the dangers of high VTF. Some would say it's better for the stylus to remain firmly seated in the groove, even at a bit too high a VTF, versus being too light and risking mis-tracking, or even bouncing around in/out of the groove. 

If I found out I'd been running a cartridge at 2.9g for (say) a week by accident, I'd correct it but wouldn't sweat the "what ifs" about whether damage may have occurred. Quite frankly I'm surprised an Ortofon 2M could hit 2.9g without bottoming out, since they're already low riders by nature and have fairly compliant suspensions. 

FWIW Ortofon's high-end MC's are spec'd to track as high as 2.5g or 2.6g, and I think some SPUs may be even beyond that.

Sure!

To enjoy music Ear is best judge. measurement is like putting thermometer to into every cm of steak for perfect temperature(and salt spice...), enjoyment is in eating the steak.