Weren't the 5770/5750 reviewed as $100/129 cards? I've seen different prices but haven't really looked at the review. AH just checked $159/129, which should be only £115-120 inc vat with a 1.6 exchange rate.
At £115, they are very good, theres a couple games it beats the 285gtx aswell as the 4980, but they are few and far between and often performance is not nearly as close, but its not nearly bad and it has VERY competitive minimum framerates compared to the 48XX series and the 260-285gtx, largely because of the increased rops, its still bandwidth limited and overclocking it, may well see it be a fantastic card as I assume as it becomes less bandwidth limited the combination of 800 ever so slightly better shaders, DX11 and most importantly, the 24 rops make it a great base raw horsepower card. But thats why its so cheap, it would seem a better deal tbh, if AMD have been a little less enthusiastic in the price war, realistically the 4890 should never have gone below £170, the 4870 never below £140 and the 4850 never below £100, which would make the 5770 a ridiculous bargain at £115-120.
I still think its the card to get and will be better and better as new games release and as dx11 drivers improve.
the 3870x2 was a great card, but right now realistically, theres just as good performing single core cards that will use a LOT less power, idle and load, less noise and just be much smaller and less issues without crossfire. I wouldn't even consider the 3870x2 now unless it was stupidly cheap, even then its a hard sell.
I'd go with the 5770, because assuming your friend doesn't upgrade much, in 2-3 years in newer games the 5770 is likely to provide better performance and a better experience than similar cards at similar price points now. You also get a very much lower power card in idle and load compared to similarly priced/performance cards from last generation.
At this point unless theres an uber deal to clear out stocks on previous AMD/Nvidia parts, I wouldn't go last gen at all.
