Does your friend have a marketing budget? I can think of some ideas that cost money, like signs on bus benches that say in huge letters "Back Pain?" and have his name very clear and easy to read.
Thinking of that, about having his name easy to read, the idea is that under the large "Back Pain?" headline should be an easy to remember contact method. So it might be a lot easier to remember a great domain name than a phone number or doctor's name or address. So the next question would be, does your friend have a great domain name, one that's easy to remember and easy to spell and that people would associate with the benefits of going to a chiropractor?
How about a listing in craigslist? Seems like the kind of thing craigslist was made for.
On the more work, less money side... I could imagine a chiropractor working up a nice little demonstration/lecture kind of presentation on how sitting with good posture and so on can decrease the risk of pains, and going to senior centers or places like that and presenting this little talk and demonstration for free, which would be helpful to people, and would help establish his expertise and credibility, and also be an opportunity for him to get publicity.
He could also do news releases related to current or seasonal issues that involve his area of expertise. It's a little late right now for snow issues, maybe, but as an example, he could issue a press release with tips for how to avoid injury while shoveling snow. Since Spring is coming the same sort of article about how to properly dig and bend and lift when doing spring cleaning or preparing a garden could be opportunities for news releases. With that in mind, the different angles on that idea, that is, spring cleaning versus gardening, could be targeted to different publications or local news segments.
He could go to businesses with lots of employees who sit all day and offer a short session in how to sit and move when using the computer for hours at a time.
How about some attractive, easy-to-read signage on his vehicle? That could be a low-cost "rolling billboard." I used to get quite a bit of my business for pooper-scooper service from people who saw my truck. I would sometimes spot them writing down my phone number at stop lights.
Post cards on bulletin boards wouldn't hurt... maybe at grocery stores (do they still have bulletin boards?) and home-improvement stores? I know I've seen bulletin boards at coffee shops and diners.
Write a nice sales letter and send it in regular first class mail to local labor-intensive services such as landscapers and contractors? Maybe use that as an introduction, maybe offer a first-time friendly neighbor discount or special offer?
A flyer in the local ads-in-a-bag that they hang on doors?
A lot depends on how much money he has to spend for ads. With money he could sponsor ads on local radio shows about home repair or gardening, things like that. Without money he could still do the PR news releases, community presentations, and other creative ideas.