Bruno,
I agree that on the face of it, it sounds too expensive for your friend. It's not too expensive for me, since I rely on it so heavily for a large number of clients, but if you're not doing this full-time, it's overkill.
That's what I used -- well the LibreOffice version, anyway, along with the file system. An invoice was just a template that I filled out, and saved to a PDF. I didn't keep every invoice as individual pages, or anything like that. Then I put the PDF in a "pending" directory, moving it into a "paid" directory when I cashed the check.
If he's just going to do the occasional invoice, it's not that hard to use one of the built-in Word templates to write one up and attach it to an email. Much more than that and I think I'd go crazy with that system.
One more thing about Freshbooks, Bruno. What quantity of invoices/clients does he plan to have at any given time? You'll notice that the free version lets you send invoices to up to three clients. Clearly this isn't really enough for most businesses (unless you have a VERY small niche), and is meant as a trial service. However, if you're doing this as a side thing, it can be enough. The reason is that once you're done invoicing a client, you can delete that client and create a new one, and it won't count against your total. All deleted clients can be restored at any time. That way you could have as many clients at a time as you wanted, but you only get three to work with at any given point.
I understand, though, if this is more complicated than it's worth. It's the reason I'm signed up for the $30/month plan (second to highest, since I don't need other people to access my invoicing). I have unlimited clients.
But yeah, I can see that it wouldn't work for him. There's another free service (or at least it was when I was using it) called Curdbee.