My rationale would be that it'd be the right thing to do since it would free this apparent nice boyfriend from the vicegrip of this devil woman. Also TS could pull it off without any harm to himself if he did it right, so why not do it?
I mean... the viable options as are would be:
1. Don't tell BF, BF pays for abortion, Gets dumped
2. Don't tell BF, BF pays for abortion, BF stays in relationship with this woman and gets screwed in who knows what ways, (Contracts STD, GETS MARRIED?!?! etc)
3. Tell BF, BF jets to greener pastures leaving abortion to be taken care by the girl/TS (moral obligation fulfilled in two ways)
Of course, the most realistic option based on TS's current moral code is:
1. Don't tell BF, BF pays for abortion, TS keeps banging the girl after abortion when the BF goes away