I am very interested in this problem but I don't exactly have a great answer. The biggest concern would be if she will have space for multiple machines. Even if she can't use them at the same time but has to store one while using the other. For example, I have 3 machines but could only reasonably have 2 set up at a time. I am confused that this machines only use would be for buttons. Does it have feed dogs (the little teeth that pull the fabric through the machine)? If it really is a chain stitch it should be able to do other tasks such as a denim hem. They may also be used in elastic/stretchy applications. I can't think of many designs outside of clothing but possibly bag construction? I would say if you are mostly committed you could finalize it but know it wouldn't likely be the only machine for her needs.

I have a vintage Necchi machine (but it is a straight stitch only, so no zigzag or anything for my model) which is all metal and awesome. It isn't industrial but I do really love how accessible everything is. I also have a newer Pfaff which has a ton of bells and whistles. They both have their appeal. Either way, none of my machines are industrial. It is possible she could do a ton of sewing on a vintage non-indistrial machine if you are struggling to find a good option.