But you CAN give your writer friend some very valuable help, and it doesn't have to cost at thing. Here are the three Rs.
OK, so you've read the e-book your friend published. Don't stop there. Sign up to Goodreads (it's free) and post a reader review. Then go to wherever you downloaded the book, and post a review there. Diesel will even pay you a small amount for the review! Reviews are very, very helpful to independent authors.
Another thing is that most of the stores don't actually require you to have purchased the book from them. You can post a review whether you bought it there or not! As far as I know, Smashwords is the only exception to this. But you still can post a review there if you download the book. Most authors will be happy to give you a coupon code for a free download of the book there if you've already bought it somewhere else, and then you can post a review there as well.
So, if you've gone to the small trouble of writing a review, get some mileage out of it!
Here are some links:
Tell your friends about the great book you are reading/just read. Tell your family (well, if it's porn maybe don't tell your mum).
When you post your review on Goodreads, use the recommend function. It will pop up right after you post your review and give you the opportunity to recommend the book to any of your friends on Goodreads. Let Goodreads post your review to Facebook too. Then share that post to any groups you belong to that may find it relevant. For example, I shared the link for David Bell's The Dog Hunters to two groups about Irish Wolfhounds. People in those groups are more than usually likely to be interested in a novel about a famous wolfhound. When you do this sensibly, you will not piss anyone off, but will rather do people a favour.
Share and retweet your friend's posts about his book, of course, and if you have a blog, you might consider posting about the book, or offering the author a guest post or interview (as I do here).
Any time you happen to be in a library or bookstore, ask for the book.
If it's a bookstore, you needn't get them to order it in - that would be dishonest if you aren't intending to buy it. It's sufficient to look very disappointed and leave without buying anything.
If it's a library, then go for it - demand they get it in. Suggest an inter-library loan. Did you know that libraries will purchase a book that readers are asking for? If you do succeed in getting the library to obtain a copy for you, borrow it immediately. When you return it, make a point of telling the librarian how great it was.
If you belong to one or more book groups, propose the book for the reading list.
I'm sure there will be many more variations on these basic tools. I'd love to hear from readers about what they did to help promote their friends' books.