Hi Vicky,
It's not quite as simple as having a 19V power supply unit for a laptop I'm afraid. The Milli Amp or Amp rating, the polarity of the connector socket, the shape of the connector socket are all considerations.
Best bet would see if you're friend can pickup a universal power supply that has multiple interchangeable connectors for different laptops, and it's power supply meets the specifications for the laptop in question. These can be had for around £25-40 (ish). Also check Ebay for any power supply units being sold off, hopefully if it's a more mainstream laptop, the replacement units won't be too expensive. If it's a more powerful laptop then expect to pay more.
Cheers! DtM! West London & Slough UK!