As no evidence exists of this contact; nothing about this encounter can make her future actions more suspicious. If Mathilde doesn't mention it, as far as anyone else is concerned it never happened.
If she does not mention it, every time in the future when something similar comes up, or a question like it (IE: have you ever hidden something that I should know re: College and Belegar) Mat will have to lie about it. That's the first part.
The second is that the enemy gets a turn too. So if a bunch of cultists start singing to the stars that Lady Magister Weber has been talking to demon lords, people might not take that accusation as truth, but there are plenty of paranoid people who'll check "just in case". And Mat will have to lie to all of them too and pass those dice rolls to deceive. Lying to Starke and Algard, likely. For now, Mat's been getting away with shit because no one is asking and they trust her. They've been asking entirely different questions and taking her as a Ranaldite, for her mistakes. Not Chaos. That changes, so do the questions. And once forbidden lore questions are asked, Mat gets to play "let's lie to the best Witch Hunters and Grey Wizards in the Empire" game, as for investigating a Lady Magister, they will send their best.
So Mat has a choice between admitting contact now, on her own terms, or the accusation coming from an unreliable source at the time her enemies find most convenient. The first means that Mat can't lie about the contact anymore, and that looks suspicious. Her game then switches to "yes we talked, but seriously, Chaos? Not even once."
Which isn't even a lie. It's a technical truth that hides all the hidden necromancy lore she has under her hat by pointing to her obvious Randald piety. Re: see pronouncement from the fucking High King on a Grudge against Ranald.
Or we let the demons choose their chosen ground and deny, deny, deny. That one means it is much harder to catch us in our first lie, but once they do, all the other ones are likely to come falling out as well.
That's why it's a devils choice and lose-lose. You can either tarnish Mat's perfect image (a little, with obvious Piety shield ready and willing to back us up vs Chaos), or put another secret under the same umbrella hiding the Liber Mortis. Where a demon already knows this secret and will tell others about it, who are our enemies.
To me, the choice is obvious. No one in the Empire will accept Mat with the Liber Mortis. That book is just too much bad juju reputation wise. So I'll rather risk looking a bit more iffy with Ranald at our backs to counter the "you met a demon" thing, then let the demon choose when its servants can attack our credibility and endanger all our secrets.
It's better to take the hit now and lean on our Piety to weather the suspicion, then allow Tzeenchina demons to choose when they want to inflict that same suspicion on us, anyway. Sure, without Mat's word the weight and push behind that suspicion will be lesser. But it's nature will be different too. It's one thing to investigate a Lady Magister that came forward and said "I met with a demon through miscast". You're looking for signs of supernatural corruption there. It's quite another thing to investigate a Lady Magister you suspect to be a hidden Black Magister, who betrayed the laws by choice.
