My gosh, Amadon1, no wonder you are worried
though, if she has capacity she can do what she wants, however foolish her actions may be
your husband can't really stop her, how would he do this? - and maybe not challenging her is the better option, so that she doesn't build up her resentment towards you both and so may accept your help further down the line
counter intuitive as it feels, you may need to just monitor her at the moment and be ready to step in if the situation hits a crisis - sorry to say, this is often how it goes
maybe you can talk with the staff at the complex and see if you can all think of what to do in the future so that you have some responses ready, rather than having to deal with something unprepared
so start looking at further support available - maybe even local care homes - see what your local AS may have to offer - and, if your MIL will allow, could your husband get some idea of her finances, ready to step in when he can
would it be possible to go with her sometimes, so that the outing is turned into a more social occasion, eg having a meal as well, rather than a full on gambling session - you may at least be able to get her home earlier
I wonder if contacting something like gambling anonymous might give you some ideas of how to support her?
best wishes