Ok, activating ultra-objective mode, no whining and no bias.
3... 2... 1... ok let's go!
Nanoha already acknowledged Tohma's innate talent for both bombardement magic and swordsmanship and of course is excited about the possibility of being the one who polish said talents. Knowing Signum, she probably be curious about said statement being a swordfighter herself, also probably she makes her own conclusions about how to fight an Eclipse Driver from her defeat at the hands of Cypha. So now that Section Six have their own eclipse driver, Signum may want to test her theories in battle and to her nothing will be better than having the strongest Eclipse Driver at her dispossal to do it.
One of three plausible things popped up in my mind about the results of this(of course, putting aside all the other biased possibilites that have Signum being curbstompped by the kid xDU) are:
a) Tohma defeats Signum(or corner her which can also be counted as a "defeat" in a sparring match) and everybody is amazed but Signum is unconvinced and tells Tohma he needs to be even better than that but thanks him for allowing her the chance to fight him.
b) Signum struggles at first but discover some important flaw in Tohma's attack pattern that allow her to defeat/corner him followed by some indication that Tohma is indeed absurdly powerfull but lacks the skill/experience to use said power to it's full potential and warns him that if she was able to find openings in his fight-style other adversaries will also be able to do so. Tohma, being the honest kid he is, will thank her for her advice.
c) Neither of them actually manage to completely disable their opponent, Signum then can take the same attitude as in the other two examples or in the contrary will congrat him for being a tough adversary and will praise Nanoha's statement about his potential. Then she'll ask Tohma for a future chance of continuing their practice, Tohma will gladly accept.
Taking into consideration the circunstances around Signum and Tohma i think she will try to do something for the kid, specially after she get to know about Tohma's past, she already was teased by Cypha about the cruel destiny that is awaiting for him as also telling her she will be unable to save him. After knowing the early hardships of his life probably she will feel identified with the boy, possibly even developing a sisterly liking of him. Naturally, Signum being Signum, she probably won't be open about this and there's even the possibility that she behave strict and cold towards him, after all, she didn't have a close relationship with him and because of this she probably decides to protect and help him from the distace, letting Subaru and the other more caring girls taking care of the emotional support.
....oooor probably i'm just talking buckets of nonsense again xDU
...."ultra-objective mode" shutting down in ...3 ...2 ...1
OFF!
Phew! i feel my head spinning xDU
EDIT: Page claimed for my first lucid post about
Signum in MONTHS!