I have to say that you and BF will both work a lot more than 50 hours a week if you own the store. There is a lot of paperwork besides just manning the front counter and loading feed into peoples cars. Someone already mentioned about the lack of documentation being a big flag for needing to know if this lady has been paying all the taxes on her business. Because if she hasn't and you take over then you can possibly become libel for any unpaid back taxes that the previous owner hasn't paid. The IRS doesn't care who was responsible for the taxes not being paid, they will just come after anyone that is associated with the business. I also get the feeling that you have probably not ever been paid with a check. That would tell me that things like insurance and state and federal taxes have not been paid. It wont hurt to check with the city to see if they have any input about you taking over this part of her business.
Another thing you want to make sure of is that the agreement you have with her is in writing. I repeat Get everything in Writing!. It won't hurt anyone, and it will save everyone a world of hurt if something goes wrong. This includes the agreement that you will have with your BF concerning the running of the business. It doesn't matter how much you love and trust the other person, it you have it in writing then there is so much less chance that something could be misunderstood, or mismanaged. You will also have to figure out how the two of you will man the store for all of the hours that it is open. I can't see how the business will support any employees anytime in the near future. Beside there are two of you to do the work.
I would also ask about what she plans to do with her side of the business when she passes. If she has no family, then it is possible that part of the business could be held in limbo, until the state figures out who might own it. It is also possible that it could be held by the state indefinitely. I am curious as to whether or not you are going to just buy her stock and take over the rental of the store, or if you are going to buy the business name and ETC also? Sometimes it is just easier to start a business from scratch, than try take over a business that has a large new debt right from the start of your ownership.
Okay I figured that is enough of the negatives about the opportunity. My first reaction to your first post was Go for it. It sounds like you can't go wrong, and that you will always regret not taking the chance. It is a great idea about opening a pet photography studio upstairs. I can see that bringing in some more income.