Nothing can be done about it now, unfortunately.
It's not possible for your friend/other people to earn the experience points from the rocks they've mined before levels for jobs(mining) were implemented into the game.
Once it came out, everyone would've needed to be on the same level to make it fair.It would've also defeated the whole idea of having a leaderboard for skill levels.
Some can argue that it's not fair for the people who mined loads of rocks - but in the end, their main purpose was to mine rocks, not level up.