Being careful is important. I am very glad that he made the attempt to suss her out before letting her join. He even got the guild masters approval of her which makes her more trustworthy in anything but a cruel world.
We all know what a "cruel world" is like after all. This doesn't have any warning signs so he should trust the master a bit. You can't run a business like a guild without trust after all, unless the world itself is cruel.