If you tap the cordless phone into the dial tone before the Magix system, as Tom points out, the Magix doesn't know when the cordless is in use.
Incoming calls will ring both the cordless and the Magix at the same time. If she answers on the cordless, the Magix will continue to ring a couple more times, until it recgonizes that the incoming ring signal has ceased. If her voice mail covers the line, it's possible that it will answer even though a call is answered on the cordless.
If her line is part of any outgoing pool accessed by other phones, she could be on a call when someone else's outgoing call grabs the line and starts dialing, interrupting the call.
The cordless only covers calls to her outside private line, NOT calls to her internal extension number. So, internal intercom calls and transferred calls from other phone numbers will not ring at the cordless phone.
By far, the best solution is to invest in a Tip/Ring card, add the cordless as its own extension, and have that extension cover her regular extension. That way, it will ring whenever the regular extension does, internal or external calls.