I would think your "friend" could go through all his various accounts and disable them, one by one. That would probably not delete his data, but it would likely reduce his visibility by a great deal.
As an example of this, consider Facebook: If you disable an account, most of what has been created using that account will become invisible (possibly deleted?), but not everything. Also, the account will still remain there, albeit invisible. If you try to rejoin later with the same e-mail address, then you will get the option of reopening the existing account instead. I would be very surprised if most other social networks and other providers of various services did not operate in similar ways; the fact is, it will be extremely difficult to ensure all your data (oh, sorry, I meant your friend's data) will really deleted.
That said, he should be able to reduce his visibility to such an extent that he will be fairly hard to find (that is, assuming your friend is not in the cross hairs of someone with a relatively high degree of competence, like Mossad, for instance).
The other part of your question is about how to initiate a new identity, and keep that separate from one's real identity. As stated by others, this has been discussed in other questions, but the following are a few pointers. (PS: I'm still learning about this stuff myself, and am not providing any guarantees. If your "friend" gets caught smuggling plutonium, or his girlfriend breaks up with him because she caught him participating in online dating because these measures turn out to be insufficiently secure, it's all on him).
- First of all, use TOR. Learn how to use it properly, and follow the general guidelines, and you should be safe enough for most cases (general anonymity).
- If you want improved anonymity, and to be sure that all your traffic is passing through TOR, get Tails Linux installed on a USB drive, and use that for all activities for your new identity.
- Never under any circumstances let your new identity interact with any accounts or anything else that you use or have accessed previously using your "previous" (or real?) identity.
- If you are really paranoid, and are worried that this is not enough, then get a separate cheap PC that you can use only for your new identity. Buy it in cash. Preferably using gloves, dark sunglasses, and a fake mustache. Only ever connect it to public networks, so it can never be linked to your address, or the address of any person or organization you may at any point have been affiliated with. Also, don't stay connected for more than a short period at a time, and keep the mustache on in public. A tinfoil hat is optional, but recommended.
Jokes aside: The real issue here is finding a balance that suits your needs. The Internet is a big place, and chances are you can "disappear in the crowd" unless someone is actively searching for you. Creating a new anonymous identity for a limited online existence is fairly easy, provided you don't actively draw attention to yourself, or have highly competent adversaries with a lot of resources. Perhaps the biggest challenge will be to find something useful to do with this new anonymous identity over an extended period of time, without interacting at any time with known friends or organizations which may give away hints to build a profile and connect the dots back to you.
Keep in mind: All kinds of usage and behavior can creates traces. A particular set of sites you visit frequently, the window size of your browser, etc. - these and other factors can be used to create a profile of your user over time. If this profile can be matched back to a similar profile for your real identity, then you may have a problem.
But then again, the odds are your friend's adversaries are not likely to be Mossad.