Sorry to break it to you but if he paid £20 for them then they will be single link DVI adapters unless he got a mega discount.
There are three types of DP-DVI adapters:
Passive, which work natively with cards that support them (usually bearing the DP++) and the cards adapts the signal. These are usually rated for 1080p 60Hz.
Active SL, which have chips that convert the signal. These are usually rated for 1920x1200 60Hz.
Active DL, which have chips that convert the signal. These are usually rated for 2560x1600 60Hz but can do higher refresh at lower res.
This man knows what he is talking about. I haven't bought an adapter in a long time and have to be honest I didn't even realise there were masses of different types. Pretty much just ignore me I think, is much safer!