Since it's a hypothetical question, here's a hypothetical idea for contemplation:
If you have a very expensive bag, to which you also have a strong emotional attachment, and you are going to a public place where people will be drinking liquids, take a bag with you that can be easily replaced, is not costly, and to which you do not have a strong emotional attachment.
We can sit here all day and talk about what the boyfriend 'should' do, and if the best friend really loves you etc etc, but the fact is that you made the decision to visit a public place where beverages would be served, it is you who are left, holding the bag, as it were, and it is you who will have to pay for cleaning the bag if it can be cleaned, or replace it if it can't, and if you do not have the money to do those things, you who will no longer have that bag.
What if neither the best friend nor the boyfriend have either anywhere near the kind of money that would be needed even to clean the bag? What if they have no idea how much the bag cost? They might offer to replace it assuming it cost $20 at Target!
You know they both work at low paying jobs and struggle to pay the rent. Are you going to tell them that no, it cost $3000, and you would like the cash by Thursday?
Maybe the best friend does know it cost $3000, and she knows neither she nor her boyfriend has it, and asks you why in the world you would sit such a bag up on the table next to a wine glass?
There are a thousand variations and combinations and ifs and shoulds and maybes.
So, you line up all your maybes and your shoulds and your ifs, and then you line up your certains, and you make your decision according to how you view the contents of your two columns!