It was over the phone. Presumably, he wasn't aware that her friends were on the extensions, soaking up every word. Kind of like those cases where the cops gather evidence by having someone the suspect trusts wear a wire and get him to run at the mouth.
Obviously, he should have insisted on a meeting in person. And no doubt should have felt her up thoroughly, if for no other reason than to make sure she wasn't wearing a wire (wires in 1982 being bulkier than today).
Did she tape record the conversation?
Can she provide any proof that these witnesses were actually there in that house at the time?
Does she have any evidence to corroborate her claim that he told her to get an abortion, that he did so over the phone, that there were witnesses present who heard this?
My guess is that all she's got is her claim and a couple of her friends who are willing to perjure themselves on her behalf.
But forget about my speculations. Here's an even better question: Why would a priest who is trying to hush up a scandal, induce an underage girl to get an abortion during a phone conversation to which anybody could be a party, during a phone conversation that, for all he knew, might actually be getting recorded on tape?
Doesn't it make more sense that he would tell her to see him in person to discuss the matter?