The airline picks up the tab for the diversion and all related activities. If the passenger doesn't have insurance to cover medical treatment at the diversion point, the airline will get the bill for that as well.

You might be surprised to find out how many passengers who wind up being the cause of medical diversions also try to seek additional damages or compensation from the airline after the fact.

It's one of those "no win" situations for an airline so they just suck up the costs and deal with it. Thankfully these occurances are not very regular.