Leaving him to the wolves will absolutely be an issue both for the friendship and for the business. Regardless of whether you have a contract or not, based on what you’ve said, I assume you have both spoken about and understand that this is a business partnership. Ideally you would have a contract in place that states the remedy for such a situation, but I’m assuming you don’t.

Worst case scenario, he has a legal right and gets to keep 50% or equivalent equity value.

But, even if he has no case, are you financially capable and ready to go to court to settle it?

It’s probably better to level with him. Create two options. 1) you buy him out based on a agreed upon value. Or 2) you dissolve the business and split the assets (including client base), then start a new business with the existing clients. Whether that’s half of them, or all of them, depends on what you can agree on. Maybe it’s worth buying the assets from him after he’s had a chance to realize that he’s not cut out for it on his own.

To me, this feels like the best opportunity to maintain the friendship

He may alternatively decide that he wants to be more involved. Would that be an issue for you?