First of all, it sounds like you're maybe enabling him, or, ... re-enforcing the false power of the thoughts, akin to worshiping a false God, giving that God more power, and only serving to strengthen the false worship. (I watched far too much Stargate :P) He or she may feel like he's or she's fine and can handle it on his or her own, but that doesn't seem to be the case, if he's or she's going to you for reassurance all the time. I know this from experience, because, unfortunately, I've done the same thing to my best mate, and my dad. :| Neither of them realised they were enabling me, ... I'm using psych-speak, but personally, I found it actually helped me, but then I think it was different, ... I know sometimes I look for reassurance, and other times I just - y'know - need to get something off my chest. I think your friend will know the difference, even if he or she will struggle to admit it.

Sometimes the thoughts and/or guilt of them, can feel so intense that you need answers or reassurance, but every time someone does that, they are only strengthening the thoughts in the first place.

Your friend really needs to get back to a doctor.

Honestly, as hard as it is to make that step to get help for OCD - this type - he or she very likely needs more than just medication, ... heck, I'm not even on medication. CBT would probably do him or her the world of good, as would perhaps a support group; these did wonders for me, in making me feel less alone, and more used to the subject of OCD, ... almost like desensitizing myself form it. Don't get me wrong, being there, understanding, and stuff, is all very awesome help, but, if at all possible, try to recognise when he's just looking for reassurance (a similar action would be ruminating, organising, cleaning, tidying, counting, touching, etc) and when he needs to just talk and not feel so alone.

If he or she has anything like what I have, he or she probably often feels very alone and possibly scared, so it's always a huge help to have someone there that is open and understanding. I love my brah to bits for how he has always been there, understanding, and welcome for me to ramble nonsense. xD Speaking of rambling.. I'm going to stop, now. :P

Oh, one more thing - as to the issue you exampled: he or she needs to... oh bollocks, I've just realised it's a he.. lol, I'm not editing all those he's, she's, hims, hers, etc. :P Anywho.. he knew it wasn't important, so that's good. The best thing I can suggest for a time like that, is distraction and some sort of grounding technique, such as using the senses like smell, touch, hearing, to ground him and bring him in this moment so as to get out of that endless loop. When I'm having a mental moment like that, anxiety is probably very high, which only makes the thoughts worse, so at that point listening to music, watching funny programmes, fiddling with textures he likes and that stick out to him, and especially using something like incense, would all aid in calming him down a bit. Oh, also, because anxiety plays such a pivotal role with OCD, it would be good to try breathing techniques, if he doesn't already.

OK, tired and my brain isn't working well, now, so that's all from moi. Hope I helped. Good luck!