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Don't worry, not interested in an "expensive V cheap" bike light thread, this is for a friend who really only needs a light once in a blue moon, so is pointless spending more than necessary. Will probably get used for Mountain Mayhem, and maybe about 2/3 more night rides per year!
So... There seems to be an incredible number of different XML based lights for sale now, from around £15 up to close to £100. Would prefer cheap as possible, but not at the expense of quality.
So what's good then? Which one to buy? Beam pattern is more important than overall (stupidly inflated!) claimed Lumen output. 3hr's or more run time would be advantageous too, though spare batteries are usually cheap for these kind of lights I understand.
If beam pattern is most important, I'd say don't get a cheap XML, they're bright but I've not seen any with a good beam pattern (they're all servicable but spotlighty) Old P7 tends to make for a better budget light.
Something like this:
http://www.ebay.co.uk/itm/SSC-P7-3-Mode-900-LM-LED-Bike-Head-Light-Lamp-Torch-/120703879782?pt=UK_SportGoods_CyclAcces_RL&hash=item1c1a830a66
I reckon spending a lot on these lights doesn't make a lot of sense, for around £80 you can get something like an MJ-872 which is a fantastic bit of kit- well made, excellent beam, mighty power. Total overkill for what you describe but then you could easily spend as much and get much less.
I would recommend one from this chap;
http://www.ebay.co.uk/sch/big_f_d_d/m.html?_nkw=&_armrs=1&_from=&_ipg=
I've had a couple from him, fantastic quality, and great customer service too. Couldn't recommend highly enough.
My recommendation is a [url= http://www.lightmalls.com/ultrafire-c8-cree-xm-l-u3-1800-lumen-5-modes-led-flashlight-1-x-18650 ]C8 XML-U3 torch[/url] which is about £15 with shipping (you need to ask for Singapore Post). You will also need a [url= http://dx.com/p/universal-bicycle-mount-for-flashlights-and-gadgets-2cm-3cm-diameter-adjustable-15642 ]mount[/url] (£2), [url= http://dx.com/p/oem-2-18650-lithium-battery-charger-110v-240v-ac-13820 ]a charger[/url] (£4), and some 18650 cells... free from an old laptop or £2.50 upwards each.
This light runs at just under 1 amp on Medium giving about 3 hours of runtime and kicks out a very useable beam of about 400-450 real lumens. It is very bright on full beam (2.4 amps... so maybe 800+ lumens) but won't last so long. I pair this with one or two XML T6 torches (on bar, on lid) which cost £8-10 each plus £3-£4 for mounts... you get a very flexible and pretty cheap set-up, and when you are not riding you have some great torches.