Hi Qatya
Welcome to the bb forum. You sound like a warm and caring person and your friend is lucky to have you.
My daughter has OCD and I have supported her over the past 9 years.
Your instincts are right, your friend needs professional mental health treatment.
The thing with OCD is that it is chronic, it never really goes away. A person can get on top of their condition (like your friend did in the past) but OCD always seems to be lurking in the background and waiting for an opportunity to strike. It is relentless and very tiring for the person who is experiencing the illness.
Your friend knows the battles that lie ahead. I suspect he may be finding it easier to let OCD in right now than to deal with it. Trouble is OCD won't stop, it will continue to ask more and more of your friend, until likely becoming disabling if left unchecked.
However, suggesting your friend to restart treatment is akin to asking him to climb a moutain blindfolded in a snowstorm. I think your message needs to be that, you know it's challenging, you know he's scared but the evidence shows that treatment can work (his past experience). Then, if it's possible, volunteer to go with him. You could even offer to make the appointment (he probably can't do that right now).
Yes, you can challenge his beliefs. But kindly and gently. You never do OCD. A simple "I'm okay with this level of contamination" will do. My only proviso with this is that depending on how unwell he is, he may find this really challenging and upsetting.
I'm almost at the word limit. Happy to keep talking and answering any more questions to support you.
Kind thoughts to you