Firstly you should not exceed the rated voltage and you have to ensure that your chargers give smoothed DC out. Secondly you need to know the polarity of the laptop socket. If I look at the back of my Toshiba laptop there is a clear symbol showing that the centre pin is positive and the outer edge is negative. If your friend's laptop has a symbol on the back and you have the appropriate size plug and you are confident that the charger you are using will supply 4.5A at 16V DC out you should be OK. But I would urge caution on cheap chargers as their outputs are often not very smooth and they sometimes have their current rating exaggerated.