The very best advice I can give is, NOT to ask them to become Catholic at all, but to ask them to say a little prayer each day that they will die united to the Church that Christ founded.
If they say, well I am already united to my Protestant belief, don't argue with them, just say, well okay then, just pray daily that you will be united to Christ's Church at the moment of your death and you will not fall away.
If they agree to pray this little pray, that will only take seconds to pray, God will hear their request and work a miracle of grace at His perfect timing.