Pros- Depends on the role you have been offered. If a senior position has been offered then I would recommend joining the firm. If a middle or junior level, then it is advisable not to join as that leaves her with little authority and control.
Secondly, it is a small organisation and has a lot to offer in terms of learning new things if the role being offered is challenging enough.
Thirdly, if you are willing to experiment, you are always join and quit whenever you feel like as the firm has no such binding obligations to remain employed with them for a long period of time.
Cons- Career growth might be extremely slow and so would be the salary increment.
Secondly, a lot of interference will always be there from the promoters in the firm and at times she might feel difficult to work in terms of independence and creativity at work.
Thirdly, biases exist (read it nepotism) so her talent or performance might get overlooked at times.