If you want to crunch for someone else there's a couple of options. First you can talk them into handing over their user name and password, then you just detach BOINC from your own account and join the project using their credentials. Couldn't be easier ... except it gives you control of their account should you choose to abuse their trust.
A safer, and just as effective option, if for them to give you their "weak key", which is a dirty great string of characters. By putting this string of characters in the right place in the right config file your system will start crunching for the other guy without giving you access to their account settings. Slightly less easy than the first method but it affords the account owner with more privacy, control of their details and prevents spectacular douchbagery on the part of the volunteer (which has been known to happen though I'd be REALLY surprised to see it here).