Isn't it in canon that Trianoma was originally a Thessalanian witch?
Shouldn't she have taught Bonisagus the details of her 'native' magic, so that it is incorporated into Hermetic Magic, like Elementalist?
Yes. See in particular HoH:S p.109 Witches of Thessaly for their magical abilities and their beginnings as worshipers of Chtonian gods.
The Major Non-Hermetic Virtue of these witches is Summoning. In HoH:S from 2007, this refers to RoP:TI p.114f Summoning: likely far too foreign and dangerous for Bonisagus to incorporate it into Hermetic magic.
Cheers
It can be argued that the use of Rego to summon various entities is what Bonisagus was able to integrate in the Hermetic Theory. Not as powerful as the Summoning ability (you cannot do "Scouring"), but still very versatile. If you look in the various book where some integration of power are proposed (Hedge magic, Ancient Magic), it is rarely a complete "copy/paste" integration, it is more like the essence of the ability is integrated, with some tweakings and adjustments.
I'd also note that one historical version of the witches has them as an astrological mystery cult.
That being said, I think there was a lot of talking with ghosts going around. Gonna the Fetid, Sybil and Archnecromantrix, was found similar stuff.
I'd have preferred she was Carolingian Chinese, myself...
I see this less as a matter of danger, and partly as Trianoma's own early political maneuverings. She makes herself primarily the political instrument of Bonisagus, going so far as to not found her own house. What Bonisagus 'gets' from her is a socially skilled magi and able to go off and gather the founders while he putters around in his proverbial lab doing important research, rather than Trianoma's own mystical knowledge necessarily. Her service to him is political, then, rather than mystical.
It's also noted in HoH:S that these same Goetic arts are also those practiced by Guorna and (assumedly) Tytalus, with Tremere probably to a lesser extent, as the heirs to her legacy. It's implied however, that the knowledge Bonisagus gains from Guorna has more to do with longevity magic than spirit-magic.
It's also known that Trianoma did have some of the powers we associate with Witches of Thessaly, but I'd look towards their guidelines in RoP:I rather than their more incorporated, current Ex Misc versions. These include a series of Virtues, one of which is Summoning - but the one most 'pointed out' in legends about Trianoma is her ability with Hexing, which is called out both in HoH:TL and Ancient Magic. It is implied that Trianoma did in fact teach Bonisagus this magic, and my guess is that it helped to build much of the modern Technique of Perdo.