Twitch gives a 50/50 split for Partners. Twitch gives people that qualify for "Partner Plus" a 70/30 split but that's only up to the first $100k each year (first $100k of net subscription revenue, so it excludes stuff like bits). To qualify for the "Partner Plus" program, you need to have 350 recurring paid subs (not gift subs) for 3 consecutive months.
If he does have Partner Plus with Twitch and assuming he hasn't touched a part of his "first $100k for the year", that means the first 20,000 subs of his subathon will push him past that limit and he'll get a 70% cut on that first 100k, so he'll get 70k. The next 30k subs would be a split of 50% and will push down his cut to 75k from that. So with a 50k subathon, he's looking at $145k altogther if he hits his subathon goal.
So if he really does owe ~$300k in taxes, I wouldn't be surprised this colossal moron doesn't understand that his twitch cut only applies to the first $100k.