As a freelance editor, I run into this quite a bit. I appear to just be sitting at home, and I am an editor, so "why not just tidy up my resume real quick," etc.
My go-to line, that I try to deliver kindly but firmly:
I'm sorry, but I really can't put my paying customers on hold to get into this for you.
If they persist (true story here):
Look, one of the reasons that you would like my help with this is because you know I take my projects seriously and try to do a good job on everything I do, right? Well, that takes time. This may seem like a quick job to you, but I promise you it won't be, not if I treat it with care. It would take a minimum of a quarter hour just for me to familiarize myself with what you already have, then we might talk for a half hour more about what you really want to accomplish with it, and even if I could spruce it up to get you what you want in 15 minutes (which is impossible), that would already be an hour of my time. Do you know I can get $40 - $75 an hour? If you don't want me to treat it special – if you just want me to look for typos – sure, fine, but we'll just do it right here on the table; get a pen and I'll circle them for you. No, girl! If I open my computer at all it'll be $20. If you want me to push other customers back and do a good job on it, you're going to have to pay me what they'd be paying me because I'll be cutting into my Paid Gig Time to do this instead..."
These may not perfectly match your situation, but I hope you can find something useful in it.
To offer something more specific, let me recap your situation: you are in a fairly new job and one of your co-workers there has already sniffed out that your skill set or expertise would benefit her husband's (or their) side company. Moreover, she talked to him about you (hence the upcoming meeting), but has not yet even told you what their company is.
If I have all that right:
- Don't be surprised if the upcoming meeting reveals it's not at all what you thought and you find yourself at a multi-level marketing meeting;
- If that's not the case, you should be able to go into this meeting not worried at all about being on the hook for anything because they haven't even told you anything; and
- I think your go-to line (after hearing them out) should be something like:
Hmmm. Yeah, this sounds really promising. But, I dunno... Look, I wouldn't mind talking things over with you over lunch now and then, but I really can't afford to sink any free time into this. Do you have any budget for this [kind of work you're looking for from me]? I mean, if my time is compensated, that would give me some more options.
Anyway, just keep it loose, honest, and polite, and don't feel like you have to say yes to anything.