Hearing aids can create all kinds of problems with phones. Simply holding the phone against the ear with the hearing aid can create unpleasant whistling and feedback for the user.
My mother has very expensive digital hearing aids that are programmed to recognize a telephone, so that significantly decreases the problem for her.
A hearing aid user can also buy a small foam rubber circle that is attached to the earpiece of the phone. That keeps the earpiece at a small distance from the ear, and that can help.
I'm not quite sure why your friend should sound garbled to you, boomerang, since the problem is really more on her end. Other than your hearing the whistling sounds from her hearing aid, I don't think the clarity of the phone transmission should be affected. But, if she has problems speaking to you, she likely has the same problem with all her phone calls.
The simplest, and easiest, solution would be for her to use the speaker phone. Since the phone isn't held against the ear, that should eliminate the problem. If her phone has speaker phone capacity ask her to try speaking with you that way.
Someone else should also try using your friend's phone in her house, to be sure the problem isn't with the phone itself or with the phone line. Garbled transmissions can be due to those factors, and would be unrelated to the hearing aids.