I know my fiancee too would not be comfortable, he would make it very clear to the man that I was taken. Me as well,if in the same situation, I would not be comfortable with it. Now though you said this is something she wanted to learn, something she wants to try or whatever. I am an artist, so I know what it is like to want to learn something new that you enjoy to do. When some one gives you the opportunity to learn such trade, you get excited and whimsical and hopeful. Some times it can be a bad thing, as this person has similar interest, if there are problems at home, besides this one, then there might be something to worry about, but if not then it probably is just what she says it is. If you make a bigger deal out of something that is nothing and start causing issues at home over it, she may end up feeling like she is not supported at home and may send her closer to some one that seems to understand. I can't say this is here, as I don't know her I just know how I would feel.
If I was your wife, and this was just something that I truely wanted to do, something I am passionate about, and you tried to keep me away from it because of your fears, It would crush me. I would probably revolt against you, saying something like " why can't you just support me, " I would feel attacked and depressed and then I would feel lonely in my indevours of the artistic field. Like you don't understand. That would then draw me closer to the person I was working with, because They understand, they listen. Now mind you, this doesn't mean this person is any better, and this doesn't mean you are not a wonderful person that does all these wonderful things, but in the moment of anger or sadness or loneliness, one tends to forget what they do have.
Here is what I propose. Perhaps make it a habbit to stop by her work unanouced from time to time when you are able, Just to relieve your worries that anything is going on. When you are there though watch how they interact with one another, that can usually give you indications of anything that could possibly be something to worry about. Ask her to make sure to call you if she is going to be late, but make sure there is an actual time when she is suppose to be out of there. If she is suppose to be off by 8 pm then have her call if for whatever reason she is going to be later than that. Perhaps ask that she be no later than a certain time as you would worry for her on her drive back. Long drives even 20 mins, things can happen. ( I used to drive about the same distance to work, in the dark, often seen deer.) Try not to smother her though, try to make sure she knows you support the things she does, that you want her to peruse her dreams, but that her alone with a man you don't know much about concerns you. Jealousy can be flattering sometimes but if you start nagging about it every day, it can just become nuisance to someone that isn't doing anything wrong. Try not to make it a bigger deal. If you start making it a daily fight, like I said, it may send her looking the other way for some one that she thinks understands her more.
If there are other issues going on at home though, where she has become more distant from you, angry with you, acts differently around you than she normally does, then you might have something to worry about.
I am not sure that I helped, but I tried to give you my view on how I see things and how I would probably react. I wish you the best of luck. Hope things work out.