If they're holding on to you and not the bike then it will be like this. Some bikes worse than others for slippy seat and no grab handles. It can help if they can get one hand on the back of the fuel tank to brace themselves against that rather than all their weight on you. You can also get some stuff called wonder web for the pillion seat that makes it really grippy so less sliding.
Yeah it was better when she was a bit more confident and held the grab rail but her arms are a bit too short to reach the tank comfortably. I’ve found a model-specific seat cover of that grippy stuff which I might try - she did say she couldn’t stop sliding fore-aft.
If they're holding on to you and not the bike then it will be like this. Some bikes worse than others for slippy seat and no grab handles. It can help if they can get one hand on the back of the fuel tank to brace themselves against that rather than all their weight on you. You can also get some stuff called wonder web for the pillion seat that makes it really grippy so less sliding.