1. Books - Depending upon your friend's photographic knowledge, books on the fundamentals of photography such as Brian Peterson's "Understanding Exposure" and Tony Northrup's "Stunning digital photography" are two very good books. Even though the 2nd book has digital in its title, the basics are the same.
Photo books from photographers that he likes are a great gift. It's critical for every photographer to study the work of past and present great photographers. Studying the past works teaches you how to be a better photographer.
2. Motor winder - These can only be found used, but what they do is more than just advance the film. Auto winders advance film by about 1 frame per second (FPS). Motor drives or motor winders advance the film up to about 5 fps. They also usually have a second shutter release located on the bottom right corner of the unit. This enables the photographer to better hold the camera when taking vertical shots.
3. Cable release. The OM-1 uses a traditional plunger type of cable release.
4. Lens cleaning kit comprised of Ziess lens cleaning solution, a microfiber cleaning cloth and a Giotto Rocket Blower.
5. Lens hood - This is to prevent lens flare when shooting into the sun. The right size will be strictly dependent upon the size fothe the filter threads. Some hoods are rubber which collapse for easier storage, while others are either metal or hard plastic.
6. Tripod - Best brands are Manfrotto, Really Right Stuff, Gitzo.
7. Flash that has a head that tilts and swivels. Look for one that has a high guide number (GN) of at least 120. The GN is not linear, so a flash that has a GN of say 80 is 2x more powerful than one that has a GN of just 56.
8. Gift card to B&H (bhphotovideo.com) or adorama.com - Both are highly reputable brick-and-mortar photo stores out of New York. I strongly recommend that you go onto their sites and check out the photo accessory section for more ideas.
9. Camera bag
10. Polarizing filter - This is essential for outdoor shooting. It removes glare and reflections from non-metallic surfaces thereby increasing details and color saturation.
11. Film - There are all sorts of B&W, color print and color slide film still on the market.
12. B&W filters - When using B&W film the resulting images will look somewhat flat in contrast if a filter wasn't used. A yellow filter creates normal contrast. Green filter will lighten green colors and darken red making a green filter really good for B&W portraits. Red filters will lighten reds and darken blues which makes the red filter excellent for landscapes where you want nice dark (or eve black) skies. Without a filter blue skies come out very pale.
13. Portrait filters - These are diffusion filters used specifically for portraits. Tiffen's FX series is among the most favored by professional portrait photographers. I've the FX-3 filter which is just awesome.
14. Lenses. Because the OM-1 system is totally abandoned by Olympus and all 3rd-party vendors, the prices for used gear has plummeted. You can get, for example, a macro lens that would cost a Nikon/Canon user $300-$500 for less than $100. Lenses like a 50mm f/1.4 are easily $400, but in the OM mount, you can find them for about $50. The 50mm f/1.4 is very popular because of it's excellent optical performance, light gathering capabilities for low-light photos, but especially because at f/1.4 it produce an extremely shallow area that's in focus (i.e. depth of field) which very, very popular.
Go to keh.com and check out their stock - https://www.keh.com/shop/lenses/slr/olym...
Keh.com always under rates the condition of their gear, the do their own repairs and they provide 6-month warranties, too. They very reputable. I've purchased almost all of my lenses from them.