While its a risk, I see little reason why the Daemon, if it wishes to not be weaponized, would not use its abilities to its fullest. Notably, Resist Psy and Strong-minded are not found on a Great Unclean one or most other daemons. The Daemonic mastery test is certainly psy/sorcery in nature, as you get to add +5 x psy rating to the mastery test, and Page 229 notes that the ritual IS modified by the target's Daemonic Presence "and other pertinent factors".
Its true that skill mastery lets you "not roll", but opposed tests go by how many DoS/DoF you have over the other, so at least we should be verifying the LoC's own, for purposes of the difference between the two. This is where strong-minded and their effective Psy Rating 9[so +45] would come in handy.
Tables aside, which in the case of summoning/binding, 201 and 229 state at the top of the tables that their maximums are +/- 60 outright rather than only implying, there's 241.
Page 241, though in the combat chapter, states: "If a situation calls for two or more bonuses or penalties, simply combine all modifiers together and apply the total to the appropriate Characteristic. The maximum total bonus that can be applied to a test is +60. Conversely, the maximum total penalty that can be applied to a test is –60. When adjudicating difficulty, common sense should prevail. Regardless of the usual limits on test penalties, some actions are simply impossible"
Personally, I'm just trying to figure out if everything went fubar for the daemon's rolls[very unlikely but certainly possible; anyone can roll a 100 sometimes, even two despite dumping infamy on there], or if the player was just led to believe that was the case.
The Daemonic mastery test might be sorcerous in nature, but it is not a psychic power, so Resistance (psychic powers) is questionable. it is most assuredly not a mind-affecting power, so Strong minded does not apply.
In addition, the fact that Daemonic mastery tests can be performed by anyone and is a pure willpower on willpower test further casts the notion that it's in any way psychic in origin into doubt. After all, this is the very same test everyone, from a Tzeentch Socerer to a Khorne Berserker, make as soon as they pick up a Daemonic weapon.
Furthermore, the text on pg 229 says "If the summoner is a psyker, he may add a +5 bonus to his test for every point of Psy Rating he has." This would imply the Daemon does not. So that +45 is out the window for the Daemon.
So let's do the math. Assume the psyker is a decent psyker of psy rating 5 and a willpower of 50. He's now up to 75. He's also Aligned to and has the Mark of Tzeentch. That's +20 and +30 respectively. 125. Assuming he has an appropriate sacrifice, he's now up to 135 vs the Daemon's 90. A strong advantage, even without him spending infamy etc for the test.
Assuming he's not 20 meters away from the Daemon (not that hard to achieve) He's brought down to 115. Give the Daemon resistance and he's now down to 105. He's got 15 on the Daemon. A less surefire bet, but even in this daemon benefiting situation, he's got a small advantage on the Daemon. If he's spent some of his (cheap) advances on willpower or has a higher psy rating (With the Mark of Tzeentch and being human, he's bought 1 level of psy rating so far) then his lead increases further.
And this is without the sorcerer doing anything in his favor. For one, if he's gonna be that close to the Daemon, why not throw up Host of Fiends, one of the cheapest powers in the game? Now he just increased his lead by 10 points, assuming he's not pushing or boon of Tzeentching it. And hey, he's Tzeentch aligned, so why not drag some useless slaves into the room and psyphon them (another cheap power). Hurl that around once or twice, and assuming average damage rolls, he gets unnatural willpower +4, for another 2 DoS of success guaranteed. That would lower the Host of Fiends a bit, unless he pushed or booned it.
So now it's 105+2 DoS versus 80. And that's with a pretty average psyker, tbh. You could reasonably achieve this within 8 or 9 sessions, tbh. Bring up a real psychic powerhouse, and this stops even being a contest.
Then we get into the actual Forbidden Lore (Daemonology) test to bind the daemon. This is not an opposed test. This is where he spent the infamy and used skill mastery. The Daemon got it's chance to oppose him in the Daemonic Mastery. Now, the best it's attempt to resist him can do is give him -30 on the test he's already stacking so many positive modifiers on it's not even funny.
As for the +60 to -60 modifier, those are listed on specific tables. So once the weapon is built, the user would use the table on 229 table to pick the proper modifiers, up to +60 or -60. And THEN apply the binding strength and psy rating. If not, a binding strength beyond 2 would be useless on a weapon wielded by anyone with the proper mark, which strikes me as improbable. And a high powered psyker would get no benefit from the mark, which is equally odd.
So, yeah, it still seems quite plausible that a Greater Daemon could be bound by a competent psyker. Now, the Master of Plotters power could throw a wrench into things. That said, I don't see how a plan could apply to a contest of wills like that, but whatever. Even then, the Heretic could hurl 2 or more infamy points into 1d5 extra DoS per point to counter that.
And this is with a pretty middling psyker. So yeah, doable. Not super easy, but the psyker in general has an advantage.