Enlightened self-interest.
Always try to make it such that there is a good reason for someone to give you help. That way, it's in their own self-interest; they see a benefit to themselves.
He already had an established relationship with the researcher, and his condition enables her to learn things that she'd never be able to otherwise.
So long as he doesn't turn evil, it's in her self-interest to aid him.
So long as his basic character doesn't change, she knows she can trust him.
He'd already decided that he could trust her, if anyone.
ETA
Having read further in the chapter...
He doesn't realize just how liked, respected, and trusted he is in this town, does he?
That was somewhat obvious previously, given his modest self-assessment and the ruminations of the Guild Advisor.
He's in for a big surprise once he actually reveals what's happened to him to a larger group, just how many will have already figured out part of what's happened, and decided to continue to trust him anyway.
And it's all justifiable based upon his back story as already presented.
This author is worth watching.
2nd ETA
Heh.
Aiding the restaurant manager should, indeed, have an impact down the road.
This is someone else, after the girl, who had no contact with him previously.
He didn't conceal his real name under an alias.
So... when the chap makes a report to the Guild on what they find, unless he's been instructed otherwise later in the story, he'll report that it was Lento who aided him.
And... restaurants are hotbeds of information exchange, are they not? After all, everyone goes to them eventually.
One of the common tropes in these settings is that useful information comes from the barkeep, etc.; how the restaurant staff talk concerning people has a great impact upon how others perceive that individual.
In other words, he's just obtained a major positive PR resource within the local community. One that will reach many who haven't had direct contact with him previously.
O course, there may be other reasons why aiding him will prove beneficial to him in the long run, given the foreshadowing at the end of this chapter.