Gee, I’ve never in person met anyone who was convinced he or she is going straight to Heaven. (Lucky them!

). But I have met other eccentrics who have some exclusive devotion to some private revelation, and are convinced that that will be like an indulgence or something. (Those people also tend to be very scrupulous.) When they try to hook me into their devotion, I just listen and smile and nod, and say “How interesting. Sounds like you have found something that really works for you and is a pathway to prayer.”
As to the Divine Mercy, if someone claimed that DM devotion was equivalent to a door to Heaven, instantly, I would merely say that no doubt they are much holier than I am, because I’m sure I will have reparations to complete still at the end of my life. Sometimes when you turn it in on yourself, people wake up and can relate to the reality that we are all sinners, and it’s incongruous to be claiming a special exception. We all deep down know our sinfulness and unworthiness, and that we cannot presume upon God’s mercy. (I would also probably use the word “presume” with her. It might ring a bill.)
But I agree with other posters: I would not engage in direct intellectual argument or contradiction (especially because she might get defensive if she’s really entrenched.) I would just confidently express my own knowledge of my sinfulness and how much I will need purification for not just recent sins but my many forgotten sins of so many years past, which I will recollect clearly once again at my Particular Judgment. (Reparation for “sins long past” might ring a bell for her because of her age.)
There’s also another real possibility, and that is that this whole “confidence” thing masks a deep anxiety about her age and the greater proximity of the end of her life. It might be a way of her coping with that anxiety by comforting herself, combined with a little denial. And if so, that would be another reason that “correcting” her is something she would find disturbing, and an effort she might vigorously work against.
Just a thought.
In fact, if I happened to know the pastor well and had an open relationship with him (if he were very approachable), I might actually say something like, “I wonder if (we’ll call her “Ann”) is beginning to be anxious about her age because she keeps talking about…” (etc.); I’m a little concerned about her and whether she’s deflecting her concerns about that. Just making you aware of this, Father; you might want to check in with her."
This would be a wonderful pastoral opportunity for him, and I can’t imagine any priest I know not seeing this as an opportunity for some pastoral initiative.
